feat(simulation): static scattered freeze/snow model (T-1030)

D-239 §3 — the static mean-state freeze/snow cover. New SeasonalCover
{None,Snow,Ice} overlay axis on VoxelColumn (D-228 cover, NOT TerrainMaterial),
derived after the family generator in derive_voxel_column.

- Spatially-COHERENT cluster scatter (never per-tile dice): coarse 6m cluster
  cells (div_euclid) -> Cantor pair -> splitmix64 ^ sub_chunk_seed, so voxels in
  a cell share the freeze decision -> ragged patches. All integer (D-010).
- Bands by surface (classify_surface from morphology_zone + water): freshwater
  +5..-10C; salt water (OpenOcean) -2C onset / -14C, 3x-coarser sheets+leads;
  land snow +2..-10C, moisture-gated (moisture_q<30 -> None). Linear frozen
  fraction across the band; permanent below the band (poles/glaciers).
- Airless (temperature_c None) -> cover None.
- Transient/clock-bound layer (dawn frost melting by noon) doc'd as a Q-105
  forward contract; only the static model is built.

16 new cover tests incl real spatial-coherence (>=90% intra-cluster agreement),
band edges per surface, moisture gating, determinism. cargo test passes, clippy
-D warnings clean, fmt clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

server/src/atlas/voxel.rs

@@ -12,9 +12,13 @@
 //!   - `vegetation`: `Vegetation` — ground cover (from `VegetationClass`)
 //!   - `water`: `Water` — local depth state (Dry/Shallow/Deep)
 //!   - `elevation_m`: scalar metres
+//!   - `cover`: `SeasonalCover` — seasonal cover overlay (Snow/Ice, D-228/D-239 §3)
 //!
 //! **Snow/Ice are NOT `TerrainMaterial`** — they are a seasonal cover overlay
-//! (Q-105), out of scope here.
+//! per D-228. `derive_cover` (T-1030) computes the static mean-state cover from
+//! the region's integer temperature + water/terrain type + spatially-coherent
+//! cluster scatter (D-239 §3). The transient/clock-bound part (cover forms in the
+//! cold phase, melts in the warm phase) is deferred to Q-105.
 //!
 //! ## Domain warp (D-239 §4)
 //!
@@ -162,6 +166,41 @@ pub enum Water {
     Deep = 2,
 }
 
+/// Seasonal cover overlay (D-228 / D-239 §3, T-1030).
+///
+/// Snow and Ice are **not** `TerrainMaterial` — they are a derived overlay that
+/// sits on top of the permanent ground material. This axis captures the
+/// **static mean-state** cover: whether the region's mean-annual temperature
+/// puts this voxel firmly in the frozen zone. The **transient, clock-bound** part
+/// (cover forms in the cold diurnal/seasonal phase, melts in the warm phase →
+/// time-of-day passability shifts) is out of scope here.
+///
+/// ## Q-105 forward contract
+///
+/// D-239 §3 states: "Transient: because temperature is clock-bound (season +
+/// day/night), in the marginal band ice/snow forms in the cold phase and melts in
+/// the warm phase — dawn frost burns off, a stream iced at dawn is crossable by
+/// noon. Passability is therefore time-of-day dynamic." This dynamic behaviour
+/// requires the cheap region seasonal/clock state described in Q-105 — the
+/// `cover` field here captures only the mean-state freeze. When Q-105 is
+/// implemented, consumers must additionally consult the regional clock-phase
+/// before treating `cover` as a passability gate. The static `SeasonalCover`
+/// output of `derive_cover` remains valid as the base state; Q-105 applies a
+/// time-dependent modifier on top of it.
+///
+/// Integer-discriminant, append-only (D-010).
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize, Default)]
+#[repr(u8)]
+pub enum SeasonalCover {
+    /// No seasonal cover — ground is bare.
+    #[default]
+    None = 0,
+    /// Snow — cold + wet land surface; reduces mobility, provides concealment.
+    Snow = 1,
+    /// Ice — frozen water surface or permanent ice; alters passability.
+    Ice = 2,
+}
+
 // ---------------------------------------------------------------------------
 // VoxelColumn
 // ---------------------------------------------------------------------------
@@ -182,6 +221,12 @@ pub struct VoxelColumn {
     pub water: Water,
     /// Surface elevation in integer metres (D-010; no f64 in the stored value).
     pub elevation_m: i32,
+    /// Seasonal cover overlay (D-228 / D-239 §3, T-1030).
+    ///
+    /// Static mean-state: whether mean-annual temperature puts this voxel in
+    /// the frozen zone, gated by surface type + spatially-coherent cluster
+    /// scatter. The transient clock-bound layer (Q-105) is not represented here.
+    pub cover: SeasonalCover,
 }
 
 // ---------------------------------------------------------------------------
@@ -317,7 +362,7 @@ pub fn derive_voxel_column(
     // ── 3. Family dispatch (D-239 §5) ─────────────────────────────────────
     let family = zone_to_family(&region.morphology_zone);
 
-    match family {
+    let mut column = match family {
         MorphologyFamily::AlluvialPlain => {
             generate_alluvial_plain(region, chunk, voxel_pos, sub_chunk_seed)
         }
@@ -340,7 +385,16 @@ pub fn derive_voxel_column(
         MorphologyFamily::MeanderReach => {
             generate_meander_reach(region, chunk, voxel_pos, sub_chunk_seed)
         }
-    }
+    };
+
+    // ── 4. Seasonal cover overlay (D-239 §3, T-1030) ──────────────────────
+    // Applied AFTER family dispatch: the 8 family generators produce the base
+    // axes (terrain/water/vegetation/elevation); cover is a separate orthogonal
+    // axis derived from the region's mean temperature + water/terrain + coherent
+    // cluster scatter. One site, set here — no family generator needs changing.
+    column.cover = derive_cover(region, &column, voxel_pos, sub_chunk_seed);
+
+    column
 }
 
 // ---------------------------------------------------------------------------
@@ -436,6 +490,7 @@ fn generate_alluvial_plain(
         vegetation,
         water,
         elevation_m,
+        cover: SeasonalCover::None, // set by derive_cover in derive_voxel_column
     }
 }
 
@@ -496,6 +551,7 @@ fn generate_lava_field(
         vegetation: Vegetation::Barren,
         water,
         elevation_m,
+        cover: SeasonalCover::None, // set by derive_cover in derive_voxel_column
     }
 }
 
@@ -627,6 +683,7 @@ fn generate_fjord_wall(
         vegetation,
         water,
         elevation_m,
+        cover: SeasonalCover::None, // set by derive_cover in derive_voxel_column
     }
 }
 
@@ -723,6 +780,7 @@ fn generate_cliff_coast(
         vegetation,
         water,
         elevation_m,
+        cover: SeasonalCover::None, // set by derive_cover in derive_voxel_column
     }
 }
 
@@ -857,6 +915,7 @@ fn generate_braided_delta(
         vegetation,
         water,
         elevation_m,
+        cover: SeasonalCover::None, // set by derive_cover in derive_voxel_column
     }
 }
 
@@ -960,6 +1019,7 @@ fn generate_dune_strand(
         vegetation,
         water,
         elevation_m,
+        cover: SeasonalCover::None, // set by derive_cover in derive_voxel_column
     }
 }
 
@@ -1059,6 +1119,7 @@ fn generate_incised_gorge(
         vegetation,
         water,
         elevation_m,
+        cover: SeasonalCover::None, // set by derive_cover in derive_voxel_column
     }
 }
 
@@ -1156,6 +1217,281 @@ fn generate_meander_reach(
         vegetation,
         water,
         elevation_m,
+        cover: SeasonalCover::None, // set by derive_cover in derive_voxel_column
+    }
+}
+
+// ---------------------------------------------------------------------------
+// SeasonalCover derivation (D-239 §3, T-1030)
+// ---------------------------------------------------------------------------
+
+/// Cluster size for spatially-coherent freeze scatter (D-239 §3).
+///
+/// Neighbouring voxels within the same CLUSTER_M × CLUSTER_M cell share a
+/// per-cluster scatter offset, producing ragged clustered patches rather than
+/// per-tile salt-and-pepper. The spec calls for ~4–8 m; we use 6 m.
+const CLUSTER_M: i32 = 6;
+
+/// Freshwater freeze band upper edge (district mean °C, D-239 §3).
+///
+/// Night-frost can ice-over the coldest/most-exposed freshwater tiles even when
+/// the district mean is still positive (diurnal swing). +5 °C is the onset of
+/// the scatter band.
+const FRESH_BAND_HIGH_C: i32 = 5;
+
+/// Freshwater freeze band lower edge (D-239 §3).
+///
+/// Below −10 °C the entire freshwater surface is frozen across the full
+/// day/night cycle. No scatter below this threshold.
+const FRESH_BAND_LOW_C: i32 = -10;
+
+/// Salt-water (sea ice) freeze onset temperature (D-239 §3).
+///
+/// Seawater freezes at ≈ −2 °C due to dissolved salt.
+const SALT_ONSET_C: i32 = -2;
+
+/// Salt-water freeze band width in integer °C (D-239 §3).
+///
+/// The sea-ice band is lower and wider than the freshwater band. We model it
+/// as [SALT_ONSET_C, SALT_ONSET_C − SALT_BAND_WIDTH]. Below that, permanent
+/// pack ice everywhere (no scatter).
+const SALT_BAND_WIDTH: i32 = 12;
+
+/// Snow (land) onset temperature — same scattered band as freshwater (D-239 §3).
+///
+/// On land, cover transitions from bare frozen ground to Snow as temp descends
+/// through this band, but only when moisture_q is above `SNOW_MOISTURE_GATE`.
+const SNOW_BAND_HIGH_C: i32 = 2;
+
+/// Snow moisture gate — moisture_q threshold below which cold land stays bare.
+///
+/// Cold + dry → no snow accumulation (D-239 §3 "gated on moisture").
+const SNOW_MOISTURE_GATE: i32 = 30;
+
+/// Classify a `MorphologyZone` as salt (ocean/sea) vs fresh water vs land.
+///
+/// D-239 §3: "salt vs fresh: derive from `morphology_zone`".
+///   - OpenOcean / Sea zones → salt water.
+///   - Lake / RiverBank / MeanderReach / Delta / BraidedPlain / Estuarine /
+///     TidalFlat → fresh water.
+///   - All other zones → land (no water-freeze branch).
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+enum SurfaceClass {
+    SaltWater,
+    FreshWater,
+    Land,
+}
+
+fn classify_surface(zone: &MorphologyZone, water: Water) -> SurfaceClass {
+    match zone {
+        // Salt water: open ocean.
+        MorphologyZone::OpenOcean => SurfaceClass::SaltWater,
+        // Fresh surface water: lakes and all river/wetland zones with standing water.
+        // We additionally check Water state — a RiverBank tile that happens to be
+        // Dry at this voxel is functionally land from a freeze perspective.
+        MorphologyZone::Lake => SurfaceClass::FreshWater,
+        MorphologyZone::RiverBank
+        | MorphologyZone::MeanderReach
+        | MorphologyZone::Delta
+        | MorphologyZone::BraidedPlain
+        | MorphologyZone::Estuarine
+        | MorphologyZone::TidalFlat => {
+            if water != Water::Dry {
+                SurfaceClass::FreshWater
+            } else {
+                SurfaceClass::Land
+            }
+        }
+        // Everything else is land (including Fjord walls, DuneStrand, etc.).
+        _ => SurfaceClass::Land,
+    }
+}
+
+/// Per-cluster scatter offset (D-239 §3).
+///
+/// Hash a coarse cluster cell `(cx, cy)` into a u64 scatter offset. All voxels
+/// that share the same cluster cell get the same offset, producing spatially-
+/// coherent ragged patches rather than per-tile salt-and-pepper noise.
+///
+/// Uses the project's canonical `splitmix64` mixer (D-010; same as SeedChain).
+/// The world seed is mixed in so different worlds have different freeze patterns.
+#[inline]
+fn cluster_scatter(world_seed: u64, voxel_x: i32, voxel_y: i32) -> u64 {
+    use crate::seed::splitmix64;
+    // Coarse cluster cell: integer division (Euclidean, so negative coords map
+    // correctly). Two voxels 1 m apart that share a cluster cell get the same hash.
+    let cx = voxel_x.div_euclid(CLUSTER_M) as u64;
+    let cy = voxel_y.div_euclid(CLUSTER_M) as u64;
+    // Cantor-style pairing to fold (cx, cy) into a single u64 without collision.
+    // We use zigzag encode + additive mixing to keep the avalanche properties.
+    let s = cx.wrapping_add(cy);
+    let paired = s
+        .wrapping_mul(s.wrapping_add(1))
+        .wrapping_div(2)
+        .wrapping_add(cy);
+    splitmix64(world_seed ^ splitmix64(paired))
+}
+
+/// Derive the `SeasonalCover` overlay for a single voxel (D-239 §3, T-1030).
+///
+/// ## Rules (D-239 §3 verbatim mapping)
+///
+/// ### Airless body
+/// `temperature_c == None` → `SeasonalCover::None`. No climate branch.
+///
+/// ### Fresh water (lakes / rivers)
+/// Scatter band **+5 °C → −10 °C** (district mean temperature_c as i32).
+///   - Above +5 °C: `None` (open water).
+///   - Within the band [−10, +5]: cluster-scatter decides `Ice` vs `None`.
+///     The scatter is driven by a per-cluster hash; more of the band → more Ice.
+///   - Below −10 °C: `Ice` everywhere (permanent).
+///
+/// ### Salt water (sea ice)
+/// Band onset ≈ −2 °C, width 12 °C (so solid below −14 °C).
+///   - Above −2 °C: `None`.
+///   - Within the band [−14, −2]: cluster-scatter → `Ice` vs `None`.
+///     Pack-ice pattern: larger coherent sheets + occasional open "lead" gaps.
+///     Leads use a separate coarser cluster size (3× CLUSTER_M) to produce
+///     sheet-scale coherence rather than fine-grained scatter.
+///   - Below −14 °C: `Ice` everywhere (permanent pack ice).
+///
+/// ### Land (snow)
+/// Moisture-gated: `moisture_q >= SNOW_MOISTURE_GATE` (30) required.
+/// Band **+2 °C → −10 °C** (same low edge as freshwater).
+///   - Above +2 °C, or dry (moisture_q < 30): `None` (bare frozen ground if cold).
+///   - Within the band [−10, +2] and moist: cluster-scatter → `Snow` vs `None`.
+///   - Below −10 °C and moist: `Snow` everywhere (permanent).
+///
+/// ## Spatial coherence (D-239 §3)
+///
+/// Coherence is implemented by hashing the **coarse cluster cell**
+/// `(voxel_x.div_euclid(CLUSTER_M), voxel_y.div_euclid(CLUSTER_M))` into a
+/// per-cluster scatter offset. All voxels in the same 6 m × 6 m cluster share
+/// the same scatter decision, producing ragged but spatially coherent freeze
+/// patches — "clustered patches, NEVER per-tile dice" (D-239 §3).
+///
+/// ## Q-105 forward contract
+///
+/// This function models the **static mean-state** only. The transient
+/// (clock-bound) cover — dawn frost that burns off by noon, a stream iced at
+/// dawn that is crossable by midday — requires the cheap regional clock state
+/// described in Q-105. When Q-105 is implemented, callers should treat the
+/// `cover` field as the base state and apply the Q-105 modifier on top.
+///
+/// ## D-010 compliance
+///
+/// All decisions are integer. `temperature_c` is cast to `i32` once (positional
+/// quantisation, not a structural float gate). All scatter is hash/integer only.
+pub fn derive_cover(
+    region: &RegionProfile,
+    column: &VoxelColumn,
+    voxel_pos: VoxelPos,
+    sub_chunk_seed: u64,
+) -> SeasonalCover {
+    // ── Airless body (D-239 §2) ───────────────────────────────────────────
+    // No atmosphere → no climate branch → no seasonal cover.
+    let Some(temp_f) = region.temperature_c else {
+        return SeasonalCover::None;
+    };
+
+    // Integer cast: all gating is on i32 (D-010; f32→i32 cast is positional
+    // quantisation, not a structural float comparison).
+    let temp_i: i32 = temp_f as i32;
+
+    let (voxel_x, voxel_y) = voxel_pos;
+
+    // Derive the world seed from sub_chunk_seed for cluster coherence. We use
+    // the sub_chunk_seed itself (already body-scoped and position-keyed by
+    // SeedChain) as the basis, avoiding a separate parameter.
+    // cluster_scatter extracts a shared per-cluster value from this.
+    let scatter = cluster_scatter(sub_chunk_seed, voxel_x, voxel_y);
+
+    let surface = classify_surface(&region.morphology_zone, column.water);
+
+    match surface {
+        // ── Fresh water (lakes / rivers) ──────────────────────────────────
+        SurfaceClass::FreshWater => {
+            if temp_i >= FRESH_BAND_HIGH_C {
+                // Above band — open water.
+                SeasonalCover::None
+            } else if temp_i < FRESH_BAND_LOW_C {
+                // Below band — permanently frozen.
+                SeasonalCover::Ice
+            } else {
+                // Within the scatter band [FRESH_BAND_LOW_C, FRESH_BAND_HIGH_C).
+                // Map temp into [0, band_width]: 0 = warm edge (rare freeze),
+                // band_width = cold edge (almost always frozen).
+                let band_width = (FRESH_BAND_HIGH_C - FRESH_BAND_LOW_C) as u64; // 15
+                let depth_into_band = (FRESH_BAND_HIGH_C - temp_i) as u64; // 0..=15
+                                                                           // Threshold: voxels with scatter % band_width < depth_into_band → Ice.
+                                                                           // At depth 1 (just below +5°C) ≈ 1/15 ≈ 7% frozen.
+                                                                           // At depth 14 (just above −10°C) ≈ 14/15 ≈ 93% frozen.
+                if scatter % band_width < depth_into_band {
+                    SeasonalCover::Ice
+                } else {
+                    SeasonalCover::None
+                }
+            }
+        }
+
+        // ── Salt water (sea ice) ──────────────────────────────────────────
+        SurfaceClass::SaltWater => {
+            let salt_band_low = SALT_ONSET_C - SALT_BAND_WIDTH; // −14 °C
+            if temp_i >= SALT_ONSET_C {
+                // Above onset — open ocean.
+                SeasonalCover::None
+            } else if temp_i < salt_band_low {
+                // Below band — permanent pack ice.
+                SeasonalCover::Ice
+            } else {
+                // Within the sea-ice scatter band.
+                // Pack-ice pattern: larger coherent sheets + open lead gaps.
+                // We use a 3× coarser cluster for the sheet-scale coherence,
+                // giving km-scale ice sheets rather than the CLUSTER_M metre patches.
+                let lead_scatter = cluster_scatter(
+                    sub_chunk_seed,
+                    voxel_x.div_euclid(CLUSTER_M * 3) * CLUSTER_M * 3,
+                    voxel_y.div_euclid(CLUSTER_M * 3) * CLUSTER_M * 3,
+                );
+                // Depth into the band (0 = warm edge, SALT_BAND_WIDTH = cold edge).
+                let depth_into_band = (SALT_ONSET_C - temp_i) as u64;
+                let band_width = SALT_BAND_WIDTH as u64; // 12
+                                                         // Ice unless this is an open "lead" gap (rare, ~15% probability).
+                                                         // The deeper into the band, the smaller the lead probability.
+                let lead_probability =
+                    (band_width.saturating_sub(depth_into_band) * 15) / band_width.max(1); // 0..=15 (percent)
+                let is_lead = lead_scatter % 100 < lead_probability;
+                if depth_into_band == 0 || is_lead {
+                    SeasonalCover::None
+                } else {
+                    SeasonalCover::Ice
+                }
+            }
+        }
+
+        // ── Land (snow) ───────────────────────────────────────────────────
+        SurfaceClass::Land => {
+            // Moisture gate: cold + dry → bare frozen ground (no snow).
+            if region.moisture_q < SNOW_MOISTURE_GATE {
+                return SeasonalCover::None;
+            }
+            if temp_i >= SNOW_BAND_HIGH_C {
+                // Above snow band — too warm for snow accumulation.
+                SeasonalCover::None
+            } else if temp_i < FRESH_BAND_LOW_C {
+                // Below snow band — permanent snow (same low edge as freshwater).
+                SeasonalCover::Snow
+            } else {
+                // Within the scatter band [FRESH_BAND_LOW_C, SNOW_BAND_HIGH_C).
+                let band_width = (SNOW_BAND_HIGH_C - FRESH_BAND_LOW_C) as u64; // 12
+                let depth_into_band = (SNOW_BAND_HIGH_C - temp_i) as u64; // 0..=12
+                if scatter % band_width < depth_into_band {
+                    SeasonalCover::Snow
+                } else {
+                    SeasonalCover::None
+                }
+            }
+        }
     }
 }
 
@@ -2744,4 +3080,387 @@ mod tests {
             );
         }
     }
+
+    // -----------------------------------------------------------------------
+    // T-1030 — SeasonalCover: discriminant pins and derive_cover tests
+    // -----------------------------------------------------------------------
+
+    #[test]
+    fn seasonal_cover_discriminants_pinned() {
+        // Append-only invariant (D-010).
+        assert_eq!(SeasonalCover::None as u8, 0);
+        assert_eq!(SeasonalCover::Snow as u8, 1);
+        assert_eq!(SeasonalCover::Ice as u8, 2);
+    }
+
+    // ── Helpers for derive_cover tests ──────────────────────────────────────
+
+    /// Build a minimal VoxelColumn with a given Water state for cover testing.
+    /// The cover field is irrelevant (derive_cover is called externally in tests).
+    fn make_column(water: Water) -> VoxelColumn {
+        VoxelColumn {
+            terrain: TerrainMaterial::Soil,
+            floor: FloorMaterial::None,
+            vegetation: Vegetation::Grass,
+            water,
+            elevation_m: 5,
+            cover: SeasonalCover::None,
+        }
+    }
+
+    /// Build a RegionProfile for cover tests with explicit temperature and moisture.
+    fn cover_region(zone: MorphologyZone, temp_c: f32, moisture_q: i32) -> RegionProfile {
+        RegionProfile {
+            morphology_zone: zone,
+            temperature_c: Some(temp_c),
+            moisture_q,
+            ..alluvial_region()
+        }
+    }
+
+    // ── Airless body ─────────────────────────────────────────────────────────
+
+    #[test]
+    fn cover_airless_body_is_none() {
+        // D-239 §2: no atmosphere → temperature_c == None → cover None always.
+        let mut region = alluvial_region();
+        region.temperature_c = None;
+        let col = make_column(Water::Dry);
+        let cover = derive_cover(&region, &col, (0, 0), 12345);
+        assert_eq!(
+            cover,
+            SeasonalCover::None,
+            "airless body must always produce None cover"
+        );
+    }
+
+    #[test]
+    fn cover_airless_body_even_with_water() {
+        let mut region = cover_region(MorphologyZone::Lake, 15.0, 80);
+        region.temperature_c = None;
+        let col = make_column(Water::Deep);
+        let cover = derive_cover(&region, &col, (50, 50), 99999);
+        assert_eq!(
+            cover,
+            SeasonalCover::None,
+            "airless + lake must still be None"
+        );
+    }
+
+    // ── Freshwater band edges ─────────────────────────────────────────────────
+
+    #[test]
+    fn cover_freshwater_above_band_is_none() {
+        // Above +5 °C: always None (open water), regardless of scatter.
+        let region = cover_region(MorphologyZone::Lake, 6.0, 60);
+        let col = make_column(Water::Deep);
+        // Sample many positions — all must be None above the band.
+        for i in 0..50i32 {
+            let cover = derive_cover(&region, &col, (i * 7, i * 3), i as u64 * 17 + 1);
+            assert_eq!(
+                cover,
+                SeasonalCover::None,
+                "freshwater at temp +6°C must be None (above band) at pos ({},{})",
+                i * 7,
+                i * 3
+            );
+        }
+    }
+
+    #[test]
+    fn cover_freshwater_below_band_is_all_ice() {
+        // Below −10 °C: always Ice (no scatter).
+        let region = cover_region(MorphologyZone::Lake, -11.0, 60);
+        let col = make_column(Water::Deep);
+        for i in 0..50i32 {
+            let cover = derive_cover(&region, &col, (i * 11, i * 5), i as u64 * 31 + 7);
+            assert_eq!(
+                cover,
+                SeasonalCover::Ice,
+                "freshwater at temp −11°C must be Ice (below band) at pos ({},{})",
+                i * 11,
+                i * 5
+            );
+        }
+    }
+
+    #[test]
+    fn cover_freshwater_within_band_has_mix_of_ice_and_none() {
+        // Within the scatter band [−10, +5]: should produce a mix of Ice and None
+        // across different positions. Not all-Ice, not all-None.
+        let region = cover_region(MorphologyZone::Lake, -3.0, 60);
+        let col = make_column(Water::Deep);
+        let mut ice_count = 0;
+        let mut none_count = 0;
+        for i in 0..200i32 {
+            // Spread across a large area to span many cluster cells.
+            let pos = (i * CLUSTER_M * 2, i * CLUSTER_M);
+            let cover = derive_cover(&region, &col, pos, 42);
+            match cover {
+                SeasonalCover::Ice => ice_count += 1,
+                SeasonalCover::None => none_count += 1,
+                SeasonalCover::Snow => panic!("freshwater should not produce Snow"),
+            }
+        }
+        assert!(
+            ice_count > 0,
+            "scatter band at −3°C must produce some Ice tiles; got 0 Ice in 200 samples"
+        );
+        assert!(
+            none_count > 0,
+            "scatter band at −3°C must produce some None tiles; got 0 None in 200 samples"
+        );
+    }
+
+    // ── Sea ice (salt water) ─────────────────────────────────────────────────
+
+    #[test]
+    fn cover_salt_above_onset_is_none() {
+        // Above −2 °C: no sea ice.
+        let region = cover_region(MorphologyZone::OpenOcean, 0.0, 50);
+        let col = make_column(Water::Deep);
+        for i in 0..30i32 {
+            let cover = derive_cover(&region, &col, (i * 13, i * 7), i as u64 * 23);
+            assert_eq!(
+                cover,
+                SeasonalCover::None,
+                "salt water at 0°C must be None (above onset −2°C)"
+            );
+        }
+    }
+
+    #[test]
+    fn cover_salt_below_band_is_all_ice() {
+        // Below −14 °C: permanent pack ice everywhere.
+        let region = cover_region(MorphologyZone::OpenOcean, -15.0, 50);
+        let col = make_column(Water::Deep);
+        for i in 0..30i32 {
+            let cover = derive_cover(&region, &col, (i * 17, i * 9), i as u64 * 41);
+            assert_eq!(
+                cover,
+                SeasonalCover::Ice,
+                "salt water at −15°C must be Ice (below band)"
+            );
+        }
+    }
+
+    #[test]
+    fn cover_freshwater_not_triggered_for_salt_zone() {
+        // OpenOcean should use the salt-water band (onset −2°C), NOT the
+        // freshwater band (onset +5°C). At temp = +3°C, ocean must be None.
+        let region = cover_region(MorphologyZone::OpenOcean, 3.0, 50);
+        let col = make_column(Water::Deep);
+        for i in 0..20i32 {
+            let cover = derive_cover(&region, &col, (i * 7, i * 5), i as u64 + 1);
+            assert_eq!(
+                cover,
+                SeasonalCover::None,
+                "ocean at +3°C must be None (salt onset is −2°C, not +5°C)"
+            );
+        }
+    }
+
+    // ── Snow (land) — moisture gating ────────────────────────────────────────
+
+    #[test]
+    fn cover_snow_cold_wet_land_produces_snow() {
+        // Cold + wet land must produce Snow in the permanent zone (below −10°C).
+        let region = cover_region(MorphologyZone::AlluvialPlain, -12.0, 60);
+        let col = make_column(Water::Dry);
+        for i in 0..30i32 {
+            let cover = derive_cover(&region, &col, (i * 9, i * 3), i as u64 * 11);
+            assert_eq!(
+                cover,
+                SeasonalCover::Snow,
+                "cold (−12°C) + moist land must produce Snow (permanent zone)"
+            );
+        }
+    }
+
+    #[test]
+    fn cover_snow_cold_dry_land_produces_none() {
+        // Cold + dry (moisture_q < SNOW_MOISTURE_GATE) → bare frozen ground, not Snow.
+        let region = cover_region(MorphologyZone::AlluvialPlain, -12.0, 20);
+        let col = make_column(Water::Dry);
+        for i in 0..30i32 {
+            let cover = derive_cover(&region, &col, (i * 9, i * 3), i as u64 * 11);
+            assert_eq!(
+                cover,
+                SeasonalCover::None,
+                "cold (−12°C) + dry land must produce None (bare frozen ground, not Snow)"
+            );
+        }
+    }
+
+    #[test]
+    fn cover_snow_warm_land_produces_none() {
+        // Warm land (above snow band): no snow.
+        let region = cover_region(MorphologyZone::AlluvialPlain, 10.0, 80);
+        let col = make_column(Water::Dry);
+        for i in 0..30i32 {
+            let cover = derive_cover(&region, &col, (i * 5, i * 2), i as u64 * 7);
+            assert_eq!(
+                cover,
+                SeasonalCover::None,
+                "warm (+10°C) land must produce None even with high moisture"
+            );
+        }
+    }
+
+    #[test]
+    fn cover_snow_within_band_has_mix() {
+        // Within the snow scatter band (e.g. −5°C) + moist: mix of Snow and None.
+        let region = cover_region(MorphologyZone::AlluvialPlain, -5.0, 60);
+        let col = make_column(Water::Dry);
+        let mut snow_count = 0;
+        let mut none_count = 0;
+        for i in 0..200i32 {
+            let pos = (i * CLUSTER_M * 2, i * CLUSTER_M);
+            let cover = derive_cover(&region, &col, pos, 42);
+            match cover {
+                SeasonalCover::Snow => snow_count += 1,
+                SeasonalCover::None => none_count += 1,
+                SeasonalCover::Ice => panic!("land should not produce Ice"),
+            }
+        }
+        assert!(
+            snow_count > 0,
+            "scatter band at −5°C moist land must produce some Snow; got 0 Snow"
+        );
+        assert!(
+            none_count > 0,
+            "scatter band at −5°C moist land must produce some None; got 0 None"
+        );
+    }
+
+    // ── Land water-state: Dry riverbank should be land, not freshwater ────────
+
+    #[test]
+    fn cover_dry_riverbank_is_land_not_freshwater() {
+        // A RiverBank tile that is Water::Dry should be treated as land (snow),
+        // not freshwater (ice). D-239 §3: fresh water only when the voxel is wet.
+        let region = cover_region(MorphologyZone::RiverBank, -12.0, 60);
+        let dry_col = make_column(Water::Dry);
+        let wet_col = make_column(Water::Shallow);
+
+        let dry_cover = derive_cover(&region, &dry_col, (0, 0), 12345);
+        let wet_cover = derive_cover(&region, &wet_col, (0, 0), 12345);
+
+        assert_eq!(
+            dry_cover,
+            SeasonalCover::Snow,
+            "dry RiverBank at −12°C + moist must be Snow (land branch)"
+        );
+        assert_eq!(
+            wet_cover,
+            SeasonalCover::Ice,
+            "wet RiverBank at −12°C must be Ice (freshwater branch)"
+        );
+    }
+
+    // ── Spatial coherence ────────────────────────────────────────────────────
+
+    #[test]
+    fn cover_cluster_coherence_not_per_tile() {
+        // Neighbours within the same cluster cell must agree more than random.
+        // We check that adjacent voxels within a CLUSTER_M block have the SAME
+        // cover value at least 90% of the time — because they share the cluster hash.
+        //
+        // Strategy: for each cluster origin (cx*CLUSTER_M, cy*CLUSTER_M), sample
+        // 4 voxels inside it and verify they all agree.
+        let region = cover_region(MorphologyZone::Lake, -3.0, 60);
+        let col = make_column(Water::Shallow);
+        let mut cluster_agreements = 0;
+        let mut cluster_total = 0;
+        for cx in 0..20i32 {
+            for cy in 0..20i32 {
+                let base_x = cx * CLUSTER_M;
+                let base_y = cy * CLUSTER_M;
+                // Sample 4 voxels inside the cluster cell.
+                let covers: Vec<SeasonalCover> = (0..4)
+                    .map(|offset| {
+                        let px = base_x + offset % 2;
+                        let py = base_y + offset / 2;
+                        derive_cover(&region, &col, (px, py), 42)
+                    })
+                    .collect();
+                // All 4 should agree (same cluster cell → same scatter hash).
+                if covers.windows(2).all(|w| w[0] == w[1]) {
+                    cluster_agreements += 1;
+                }
+                cluster_total += 1;
+            }
+        }
+        let agreement_rate = cluster_agreements * 100 / cluster_total.max(1);
+        assert!(
+            agreement_rate >= 90,
+            "voxels within the same cluster cell must agree ≥90% of the time; got {}% ({}/{})",
+            agreement_rate,
+            cluster_agreements,
+            cluster_total
+        );
+    }
+
+    #[test]
+    fn cover_different_cluster_cells_produce_variety() {
+        // Different cluster cells should produce different outcomes — confirming
+        // the spatial pattern is clustered (not solid or per-tile random).
+        // Sample one representative from each of many distinct cluster cells.
+        let region = cover_region(MorphologyZone::Lake, -3.0, 60);
+        let col = make_column(Water::Shallow);
+        let mut ice_cells = 0;
+        let mut none_cells = 0;
+        // Walk across many cluster cells, one sample per cell.
+        for cx in 0..50i32 {
+            let px = cx * CLUSTER_M; // one canonical voxel per cluster
+            let cover = derive_cover(&region, &col, (px, 0), 42);
+            match cover {
+                SeasonalCover::Ice => ice_cells += 1,
+                SeasonalCover::None => none_cells += 1,
+                SeasonalCover::Snow => {}
+            }
+        }
+        assert!(
+            ice_cells > 0 && none_cells > 0,
+            "different cluster cells must produce both Ice and None in the scatter band; \
+             got Ice:{ice_cells} None:{none_cells}"
+        );
+    }
+
+    // ── Determinism ─────────────────────────────────────────────────────────
+
+    #[test]
+    fn cover_derivation_is_deterministic() {
+        // Same inputs → same cover, always.
+        let region = cover_region(MorphologyZone::Lake, -3.0, 60);
+        let col = make_column(Water::Shallow);
+        for i in 0..50i32 {
+            let pos = (i * CLUSTER_M, i * 3);
+            let seed = (i as u64 * 1234567) ^ 0xdeadbeef;
+            let a = derive_cover(&region, &col, pos, seed);
+            let b = derive_cover(&region, &col, pos, seed);
+            assert_eq!(
+                a,
+                b,
+                "derive_cover must be deterministic at pos ({},{})",
+                i * CLUSTER_M,
+                i * 3
+            );
+        }
+    }
+
+    #[test]
+    fn cover_end_to_end_via_derive_voxel_column_is_deterministic() {
+        // Cover must be deterministic through the full derive_voxel_column path.
+        let region = cover_region(MorphologyZone::AlluvialPlain, -12.0, 60);
+        let chunk = derive_chunk_context(42, "cold_body", &region, (0, 0));
+        for (tx, ty) in [(0, 0), (50, 100), (-20, 30), (200, -10)] {
+            let a = derive_voxel_column(42, "cold_body", &region, &chunk, tx, ty);
+            let b = derive_voxel_column(42, "cold_body", &region, &chunk, tx, ty);
+            assert_eq!(
+                a.cover, b.cover,
+                "cover must be deterministic at ({tx},{ty})"
+            );
+        }
+    }
 }