feat(simulation): cache TerrainAnalysis + derive basin_direction from the D8 thalweg (T-1044, T-1047)

T-1044: run_layer1 now returns TerrainAnalysis (carried transiently on
CascadeSnapshot, dropped after the district + road-graph passes), eliminating the
redundant per-body drainage::analyze + TerrainAnalysis::analyze re-run flagged by
PERF/TODO(T-1044). Not persisted on the LRU-cached state (D-203/T-1048 size concern).

T-1047: basin_direction is now derived from the real D8 thalweg. run_layer1
aggregates a per-district dominant D8 direction from the live fdir grid (carried
transiently on DrainageResult), threaded via Layer1Output.district_basin_dirs ->
derive_all_districts -> DistrictProfile.basin_direction; derive_chunk_context reads
it directly. Removed the false derive_basin_direction (it branched on ocean_fraction_q
then read seed bits despite a doc comment claiming an elev_q/slope_q D8 proxy) +
corrected the module contract. D-239 §8 (D8 thalweg) now actually honoured.

1559 tests pass; golden byte-identical (district_basin_dirs is #[serde(skip)], transient).

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

server/src/atlas/chunk_context.rs

@@ -10,7 +10,10 @@
 //! **coarser than a chunk** — so there is no per-tile `flow_direction[64×64]`
 //! here. Instead the chunk carries:
 //!   - **`basin_direction`** — a cardinal basin-flow direction (N/E/S/W, 4-way)
-//!     derived from the dominant D8 thalweg in the covering district.
+//!     sourced directly from `DistrictProfile.basin_direction`, which holds the
+//!     **true D8-computed dominant thalweg direction** aggregated in `run_layer1`
+//!     (T-1047, D-239 §8). Prior to T-1047 this was incorrectly derived from
+//!     seed bits; the false derivation (`derive_basin_direction`) has been removed.
 //!   - **`meander_phase`** and **`meander_wavelength_m`** — global meander-curve
 //!     params for the MeanderReach and AlluvialPlain families.
 //!
@@ -55,28 +58,11 @@ use crate::atlas::district_profile::DistrictProfile;
 use crate::atlas::scale;
 use crate::seed::{SeedChain, SeedDomain};
 
-// ---------------------------------------------------------------------------
-// Basin direction (cardinal, 4-way)
-// ---------------------------------------------------------------------------
-
-/// Cardinal basin-flow direction — the dominant D8 thalweg direction in the
-/// covering district. Coarser than a chunk (D-239 §10); derived from district slope
-/// and morphology, NOT from a per-tile D8 grid.
-///
-/// Integer-discriminant, append-only (D-010).
-#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize, Default)]
-#[repr(u8)]
-pub enum BasinDirection {
-    /// Flow toward the north (decreasing y in grid coords).
-    #[default]
-    North = 0,
-    /// Flow toward the east (increasing x in grid coords).
-    East = 1,
-    /// Flow toward the south (increasing y in grid coords).
-    South = 2,
-    /// Flow toward the west (decreasing x in grid coords).
-    West = 3,
-}
+// Re-export so existing consumers (voxel.rs etc.) keep their `chunk_context::BasinDirection` path.
+// The canonical definition lives in `scale.rs` (T-1047) — `BasinDirection` is shared
+// between `DistrictProfile` (district_profile.rs) and `ChunkContext` (this module),
+// and placing it in the scale ladder breaks the potential circular import.
+pub use crate::atlas::scale::BasinDirection;
 
 // ---------------------------------------------------------------------------
 // ChunkPos — position on the 64 m chunk grid
@@ -223,10 +209,18 @@ pub struct ChunkContext {
 ///
 /// ## Seed usage
 ///
-/// The **meander phase** and **basin direction** are seeded at district scale so
-/// features with wavelength > 64 m are consistent across chunk boundaries.
-/// The chunk-local seed (keyed on `chunk_pos`) is reserved for sub-chunk scatter
-/// in the voxel pass — not consumed here.
+/// The **meander phase** is seeded at district scale so features with
+/// wavelength > 64 m are consistent across chunk boundaries. The chunk-local
+/// seed (keyed on `chunk_pos`) is reserved for sub-chunk scatter in the voxel
+/// pass — not consumed here.
+///
+/// ## Basin direction
+///
+/// `basin_direction` is read directly from `district.basin_direction` — the
+/// **true D8-computed dominant thalweg direction** threaded from `run_layer1`
+/// (T-1047, D-239 §8). No seed-bit proxy is used here. The former
+/// `derive_basin_direction` helper (which branched on `ocean_fraction_q` and
+/// seed bits, not on the actual D8 grid) has been removed.
 pub fn derive_chunk_context(
     world_seed: u64,
     body_id: &str,
@@ -246,11 +240,10 @@ pub fn derive_chunk_context(
     let district_seed = SeedChain::for_body(world_seed, body_id)
         .derive(SeedDomain::ChunkContext, district_scale_id);
 
-    // Basin direction — derived from district slope_q (which encodes the
-    // dominant terrain gradient). We use the district's `elev_q` gradient
-    // direction as a proxy for the D8 thalweg direction.
-    // All integer arithmetic (D-010).
-    let basin_direction = derive_basin_direction(district, district_seed.seed());
+    // Basin direction — read directly from DistrictProfile (T-1047, D-239 §8).
+    // This is the true D8-computed dominant thalweg direction aggregated in
+    // run_layer1 from the fdir grid; it replaces the former seed-bit proxy.
+    let basin_direction = district.basin_direction;
 
     // Meander phase — district-scale integer offset so the channel is consistent
     // across all chunks in the same district. 0–255.
@@ -450,38 +443,6 @@ pub(crate) fn pos_to_id(pos: (i32, i32)) -> u64 {
         .wrapping_add(y)
 }
 
-/// Derive basin direction from the district profile and a district-scale seed.
-///
-/// Uses `elev_q` and `slope_q` as a proxy for the dominant D8 gradient
-/// direction. In lieu of a full D8 computation at this scale, the basin
-/// direction is derived from the district's terrain characteristics:
-///   - Coastal districts (high `ocean_fraction_q`) flow toward ocean (West fallback)
-///   - High-elevation districts flow away from ridges (seed-derived direction)
-///   - Low-slope districts use the seed for unbiased direction
-///
-/// All integer arithmetic (D-010).
-fn derive_basin_direction(district: &DistrictProfile, district_seed: u64) -> BasinDirection {
-    // Coastal: flow toward the ocean (use seed to pick E/W/N/S with coastal bias).
-    if district.ocean_fraction_q >= 15 {
-        // The low 2 bits of seed give 4 directions; bias toward the most common
-        // coastal configurations (N or S for equatorial coasts, E/W for shelf).
-        return match (district_seed >> 2) & 0x3 {
-            0 => BasinDirection::South,
-            1 => BasinDirection::East,
-            2 => BasinDirection::North,
-            _ => BasinDirection::West,
-        };
-    }
-
-    // Interior: pure seed-derived direction (unbiased).
-    match district_seed & 0x3 {
-        0 => BasinDirection::North,
-        1 => BasinDirection::East,
-        2 => BasinDirection::South,
-        _ => BasinDirection::West,
-    }
-}
-
 /// Derive meander wavelength in metres from district morphology.
 ///
 /// Low slope + high moisture → longer wavelength (wide meanders).
@@ -543,6 +504,7 @@ mod tests {
             temperature_c: Some(18.0),
             moisture_q: 55,
             vegetation_class: VegetationClass::Forest,
+            basin_direction: BasinDirection::South,
         }
     }
 
@@ -679,6 +641,7 @@ mod tests {
             temperature_c: Some(35.0),
             moisture_q: 5,
             vegetation_class: VegetationClass::Barren,
+            basin_direction: BasinDirection::North,
         };
         let ctx = derive_chunk_context(42, "dry_body", &district, (5, 5), None);
         assert!(