feat(simulation): voxel-tier mid-scale relief — navigable hills (T-1081)

The voxel tier read flat (the D-245 "0–3 m, no hills to navigate by" bug):
family generators set elevation from elev_q at a compressed scale plus only ±4 m
micro-scatter, and detail-scatter's mid-scale relief (D-243 §2) only ever reached
the district tier (elev_q), never per-voxel elevation_m.

- detail_scatter.rs: extract the enveloped-fBm core; add `voxel_relief` at the
  0.25–2 km octave band (vs the district 4–40 km band).
- voxel.rs: add the mid-scale relief post-dispatch in derive_voxel_column, body-global
  SeedDomain::VoxelRelief seed (position-keyed), f64 truncated to integer metres (D-010).
  Envelope = slope_q*3 + elev_q — the coarse heightmap gives slope_q ≈ 0 even on high
  ground (Arbour max 13), so elevation must drive ruggedness. Flat families only
  (Alluvial/Lava/BraidedDelta/Dune/Meander); the dramatic families already carry strong
  internal relief and WaterBody stays at sea level.
- seed.rs: SeedDomain::VoxelRelief (D-224 domain separation).
- believability.rs: new `contrast.voxel_relief_m` metric — mean within-district elevation
  range over a 2 km transect (a single 64 m sample chunk is narrower than the relief
  band) + a "voxel relief" criterion.

Arbour @ yolo: voxel relief 5 m → 59 m, 6/8 → 7/8 criteria (remaining fail = the
Q-123 vegetation-denominator item). Edict 59 m. Golden regenerated; 1586 lib tests +
believability harness (determinism) pass; clippy -D warnings clean.

D-239 amended (T-1081); Q-123 item 4 updated. Deferred to Q-123/follow-up: the
provisional span + threshold, and a per-body hypsometric absolute-elevation model.

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

server/tests/golden/believability.json

@@ -27,7 +27,8 @@
       },
       "morphology_zones": 9,
       "vegetation_classes": 3,
-      "terrain_materials": 4
+      "terrain_materials": 4,
+      "voxel_relief_m": 49
     },
     "coherence": {
       "water_districts": 49,
@@ -66,7 +67,8 @@
       },
       "morphology_zones": 9,
       "vegetation_classes": 1,
-      "terrain_materials": 4
+      "terrain_materials": 4,
+      "voxel_relief_m": 45
     },
     "coherence": {
       "water_districts": 25,