fix(simulation): unify C1 climate gate across Wetland+Forest+Grassland (PR #172 N1)

Tyre's re-review found the C1 !Barren guard only covered the Grassland arm — the Wetland arm was an unconditional => true. Since derive_morphology_zone has no temperature gate, a frozen + wet + flat district still yields Wetland material, so the wetland palette would paint copse/bog onto frozen marsh: the exact C1 symptom via the material path. Not a regression (the arm was unchanged) and unreachable by the 2 harness bodies, but the D-246 amendment + the voxel.rs comment already claim the guarantee universally, so the gate must match the claim.

Gate is now: apply = !Barren && class in {Wetland, Forest, Grassland}. Behavior change is limited to frozen/hyper-arid Wetland districts (a temperate marsh is never Barren-dominant; Forest is Barren-free by construction). believability.json unchanged (neither harness body reaches the frozen-wet case). New regression test mosaic_pass_does_not_revegetate_frozen_wetland; D-246 amendment reworded to the unified gate. Full cargo test green; clippy -D warnings clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server/src/atlas/voxel.rs

@@ -537,19 +537,24 @@ pub fn derive_voxel_column(
         // boreal / tundra read distinctly (D-246 biome-awareness).
         let biome =
             mosaic::ClimateBiome::from_climate(district_eff.temperature_c, district_eff.moisture_q);
-        let apply = match class {
-            SurfaceClass::Wetland | SurfaceClass::Forest => true,
-            // Only where the climate already grants cover (C1 fix). A Barren-dominant
-            // district is Barren for a reason `derive_vegetation` already resolved —
-            // frozen (< -50 °C = surface ice/geology, D-239 §2), hyper-arid (moisture < 5),
-            // or cold-dry marginal ground — so the mosaic must NOT re-grant it (else it
-            // paints tundra scrub onto frozen ice worlds like Edict). Genuine tundra
-            // (cold + moisture ≥ 15) is already Scrub upstream, so it reaches the biome's
-            // tundra palette through this same !Barren path — no special cold clause needed.
-            SurfaceClass::Grassland => !matches!(dominant_veg, Vegetation::Barren),
-            // Rock / Sand / Gravel / Lava are material-driven — a §8-aware follow-up.
-            _ => false,
-        };
+        // Apply within the vegetated / wet land classes — but ONLY where the climate
+        // already grants cover (C1 + N1). A Barren-dominant district is Barren for a
+        // reason `derive_vegetation` already resolved — frozen (< -50 °C = surface
+        // ice/geology, D-239 §2), hyper-arid (moisture < 5), or cold-dry marginal ground
+        // — so the mosaic must NOT re-grant it, else it paints tundra scrub onto a frozen
+        // ice world (Edict, via the Grassland arm) or wetland copse onto a frozen marsh
+        // (via the Wetland arm — `derive_morphology_zone` has no temperature gate, so
+        // frozen + wet + flat still yields Wetland material). The `!Barren` guard therefore
+        // spans all three classes, not just Grassland. Genuine tundra (cold + moisture ≥ 15)
+        // is Scrub upstream (not Barren), so it still reaches the biome's tundra palette.
+        // Forest is Barren-free by construction (SurfaceClass::Forest requires
+        // dominant_veg == Forest). Rock / Sand / Gravel / Lava are material-driven — a
+        // §8-aware follow-up (D-246 amendment).
+        let apply = !matches!(dominant_veg, Vegetation::Barren)
+            && matches!(
+                class,
+                SurfaceClass::Wetland | SurfaceClass::Forest | SurfaceClass::Grassland
+            );
         if apply {
             let mosaic_seed = SeedChain::for_body(world_seed, body_id)
                 .derive(SeedDomain::VoxelMosaic, 0)
@@ -2629,6 +2634,35 @@ mod tests {
         }
     }
 
+    /// N1 regression (PR #172 re-review): the C1 climate gate must hold on the Wetland
+    /// arm too, not only Grassland. `derive_morphology_zone` takes no temperature, so a
+    /// FROZEN + wet + flat district maps to Wetland material (`SurfaceClass::Wetland`)
+    /// while `derive_vegetation` returns Barren — the exact C1 symptom via the material
+    /// path. The unified `!Barren` gate skips it, so the frozen marsh stays Barren rather
+    /// than being painted with the wetland palette (copse / meadow / bog / marsh). Before
+    /// the N1 fix the Wetland arm was an unconditional `=> true`.
+    #[test]
+    fn mosaic_pass_does_not_revegetate_frozen_wetland() {
+        let frozen_wet = DistrictProfile {
+            vegetation_class: VegetationClass::Barren,
+            temperature_c: Some(-60.0), // frozen: surface ice = geology (D-239 §2)
+            moisture_q: 70,             // ≥ 60 + flat ⇒ Wetland material (SurfaceClass::Wetland)
+            morphology_zone: MorphologyZone::Wetland,
+            slope_q: 3,
+            ..alluvial_district()
+        };
+        let chunk = derive_chunk_context(42, "frozenmarsh", &frozen_wet, (10, 20), None);
+        for tx in [100, 137, 200, 263, 331] {
+            let col = derive_voxel_column(42, "frozenmarsh", &frozen_wet, &chunk, tx, 90);
+            assert_eq!(
+                col.vegetation,
+                Vegetation::Barren,
+                "frozen Wetland district must stay Barren (N1), got {:?} at x={tx}",
+                col.vegetation,
+            );
+        }
+    }
+
     #[test]
     fn elevation_is_non_negative() {
         let district = alluvial_district();