feat(simulation): intra-class micro-habitat mosaic v1 — vegetation/water texture (T-1084, D-246)

Sub-chunk mosaic that modulates the D-228 axes into a spatially-coherent,
relief/moisture-conditioned mosaic of micro-habitats within a terrain class —
the fix for the aliveness probe's uniform "Wetland 100%".

- seed.rs: SeedDomain::VoxelMosaic = 12 (append-only; pinned-discriminant guard)
- voxel.rs: five new D-228 axis values (Vegetation::{Meadow,Deadfall,Lichen},
  TerrainMaterial::{Hardpan,Scree}); the per-family-gated mosaic pass
- detail_scatter.rs: voxel_mosaic() selection field (~8-64 m band)
- atlas/mosaic.rs: surface-class palette + integer-basis-point weighted selection
  with moisture-affinity conditioning; temperate/native baseline palettes
- data/mosaic_constants.toml: source-canonical palette mirror
- believability.rs: micro_habitat_distinct metric + D-245 intra-class criterion

v1 scope (D-246-faithful): modulates vegetation + water only, in the vegetated
classes (Wetland/Forest/Grassland), so the D-239 §8 lithology law is untouched;
water is gated on the T-1040 has_active_channel hydrology rule. Material variants
(hardpan/scree) and the extreme classes are a §8-aware follow-up; the biome-aware
palette variants (T-1085 vocab) extend the baseline before generation.

Goldens regenerated (believability + derivation). Full server suite green.

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

server/tests/golden/believability.json

@@ -28,7 +28,8 @@
       "morphology_zones": 9,
       "vegetation_classes": 3,
       "terrain_materials": 4,
-      "voxel_relief_m": 20
+      "voxel_relief_m": 20,
+      "micro_habitat_distinct": 2
     },
     "coherence": {
       "water_districts": 49,
@@ -68,7 +69,8 @@
       "morphology_zones": 9,
       "vegetation_classes": 1,
       "terrain_materials": 4,
-      "voxel_relief_m": 19
+      "voxel_relief_m": 19,
+      "micro_habitat_distinct": 0
     },
     "coherence": {
       "water_districts": 25,

server/tests/golden/derivation_harness.json

@@ -6,7 +6,7 @@
     "tile_x": 1375,
     "tile_y": 800,
     "terrain": 0,
-    "vegetation": 4,
+    "vegetation": 8,
     "water": 0,
     "elevation_m": 14,
     "cover": 0