fix(simulation): make voxel relief sub-district + fix integration-test fallout (T-1081)

The push gate's integration tests (not in cargo test --lib) caught two regressions
from the relief change:

- derivation_harness::cross_district_elevation_blend_reduces_seam_step — the relief's
  large per-position term (span 300 → ±200 m, far above the compressed elev_q/2 base)
  swamped the T-1042 seam measurement and clamped heavily at sea level. Fixes:
  (1) shift the voxel-relief octave band to 0.13–1 km (all sub-district, dropping the
  2 km octave that competed with elev_q's district role); (2) reduce VOXEL_RELIEF_SPAN_M
  300 → 100 so relief stays mostly below the base (fewer sea-level clamp artifacts,
  proportional hills); (3) rewrite the seam test's avg_elev to average over an 8 km
  multi-wavelength y-transect so the zero-mean relief cancels, isolating the base seam.
- derivation_harness::golden_seed_determinism_regression — legitimate golden refresh
  (determinism still holds; elevation values changed intentionally).

Believability relief now ≈22 m mean (was 5 m flat; 59 m at the over-aggressive span 300)
— navigable hills without clamp artifacts, 7/8 criteria. Both goldens regenerated; full
cargo test (38 binaries) + clippy --all-targets -D warnings green. D-239 amendment +
Q-123 item 4 updated to the final span/octave/numbers.

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

server/tests/derivation_harness.rs

@@ -1191,40 +1191,42 @@ fn cross_district_elevation_blend_reduces_seam_step() {
         "T-1042: boundary blend weight must be 128 (50-50)"
     );
 
-    // Derive ChunkContext for last-A with the secondary district B at 50-50 blend.
-    let ctx_last_a = derive_chunk_context(
-        seed,
-        body,
-        &district_a,
-        last_a_chunk,
-        Some((&district_b, blend_w)),
-    );
-    // First-B: no blend (interior to B).
-    let ctx_first_b = derive_chunk_context(seed, body, &district_b, first_b_chunk, None);
-    // Interior chunks: no blend.
+    // Interior-A context retained for the determinism sub-check (Criterion 3).
     let ctx_interior_a = derive_chunk_context(seed, body, &district_a, interior_a_chunk, None);
-    let ctx_interior_b = derive_chunk_context(seed, body, &district_b, interior_b_chunk, None);
 
-    // Average elevation across a full 64-voxel row through each chunk.
-    // We scan y=0 (along the x cross-axis for this AlluvialPlain basin).
-    let avg_elev = |chunk_pos: ChunkPos,
-                    ctx: &settled_reach_server::atlas::chunk_context::ChunkContext,
-                    dist: &DistrictProfile|
-     -> i64 {
-        let base_x = chunk_pos.0 * scale::CHUNK_M;
-        let base_y = chunk_pos.1 * scale::CHUNK_M;
-        let mut sum = 0i64;
-        for dx in 0..scale::VOXELS_PER_CHUNK {
-            let col = derive_voxel_column(seed, body, dist, ctx, base_x + dx, base_y);
-            sum += col.elevation_m as i64;
-        }
-        sum / scale::VOXELS_PER_CHUNK as i64
-    };
+    // Average elevation over a tall multi-wavelength y-transect at a given x-column,
+    // deriving a ChunkContext per y-chunk. T-1081 adds a zero-mean voxel-relief term to
+    // elevation_m; averaged over ≥4× the coarsest relief wavelength (1024 m) it cancels,
+    // leaving the elev_q-derived base — the quantity T-1042's blend actually smooths. A
+    // single 64 m row would carry a per-chunk relief offset that swamps the seam signal.
+    let avg_elev =
+        |x_chunk: i32, blend: Option<(&DistrictProfile, u8)>, dist: &DistrictProfile| -> i64 {
+            const Y_CHUNKS: i32 = 128; // 8192 m ≈ 8× the 1024 m coarsest relief octave
+            let base_x = x_chunk * scale::CHUNK_M;
+            let mut sum = 0i64;
+            let mut n = 0i64;
+            for yc in 0..Y_CHUNKS {
+                let cp: ChunkPos = (x_chunk, yc);
+                let ctx = derive_chunk_context(seed, body, dist, cp, blend);
+                let base_y = yc * scale::CHUNK_M;
+                for dx in (0..scale::VOXELS_PER_CHUNK).step_by(8) {
+                    for dy in (0..scale::VOXELS_PER_CHUNK).step_by(8) {
+                        let col =
+                            derive_voxel_column(seed, body, dist, &ctx, base_x + dx, base_y + dy);
+                        sum += col.elevation_m as i64;
+                        n += 1;
+                    }
+                }
+            }
+            sum / n
+        };
 
-    let elev_last_a = avg_elev(last_a_chunk, &ctx_last_a, &district_a);
-    let elev_first_b = avg_elev(first_b_chunk, &ctx_first_b, &district_b);
-    let elev_interior_a = avg_elev(interior_a_chunk, &ctx_interior_a, &district_a);
-    let elev_interior_b = avg_elev(interior_b_chunk, &ctx_interior_b, &district_b);
+    // Force the 50-50 blend with B across the whole last-A column (x=31); the interiors
+    // and first-B column are unblended (per the boundary detection asserted above).
+    let elev_last_a = avg_elev(last_a_chunk.0, Some((&district_b, blend_w)), &district_a);
+    let elev_first_b = avg_elev(first_b_chunk.0, None, &district_b);
+    let elev_interior_a = avg_elev(interior_a_chunk.0, None, &district_a);
+    let elev_interior_b = avg_elev(interior_b_chunk.0, None, &district_b);
 
     // Seam step = elevation gap between the blended last-A chunk and the clean first-B chunk.
     let seam_step = (elev_last_a - elev_first_b).unsigned_abs() as i64;

server/tests/golden/believability.json

@@ -28,7 +28,7 @@
       "morphology_zones": 9,
       "vegetation_classes": 3,
       "terrain_materials": 4,
-      "voxel_relief_m": 49
+      "voxel_relief_m": 20
     },
     "coherence": {
       "water_districts": 49,
@@ -68,7 +68,7 @@
       "morphology_zones": 9,
       "vegetation_classes": 1,
       "terrain_materials": 4,
-      "voxel_relief_m": 45
+      "voxel_relief_m": 19
     },
     "coherence": {
       "water_districts": 25,

server/tests/golden/derivation_harness.json

@@ -20,7 +20,7 @@
     "terrain": 5,
     "vegetation": 0,
     "water": 0,
-    "elevation_m": 21,
+    "elevation_m": 36,
     "cover": 0
   },
   {