feat(simulation): grassland biome-awareness + alien-ecology vocab (T-1085 workstream 4, D-246)

The mosaic palette gains a climate-biome dimension for the Grassland surface-class:
savanna / temperate / boreal / tundra now read distinctly rather than collapsing to
one biome-blind palette (the D-246 biome-awareness fix).

- atlas/mosaic.rs: ClimateBiome (from temperature_c + moisture_q) + four biome-keyed
  Grassland palettes; palette(class, biome). Savanna carries scattered Forest copses,
  tundra carries lichen + sparse bare ground and no forest. §8-safe (veg + water only).
- atlas/voxel.rs: derive the biome; a cold barren district (tundra/boreal) now takes
  the sparse mosaic rather than staying blank — worlds that read "0 vegetated / blank"
  now read structured-sparse (D-245 climate-appropriateness). vegetated_districts
  8→20 and 0→39 on the cold validation bodies.
- data/mosaic_constants.toml: biome-keyed grassland mirror.
- docs/design/alien-ecology-vocabulary.md: Miri's full biome × register naming layer
  (the cane/vane/wrack/husk/drum-root native lexicon + Earth-register names) — the
  derived-label source for the palette and Phase-5 rendering.

Believability golden regenerated. Full server suite green. Forest biomes and the
introduced-Earth register are the noted continuation.

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

server/tests/golden/believability.json

@@ -37,7 +37,7 @@
       "drainage_samples": 0,
       "drainage_monotonic": 0,
       "vegetation_samples": 64,
-      "vegetated_districts": 8
+      "vegetated_districts": 20
     }
   },
   {
@@ -70,7 +70,7 @@
       "vegetation_classes": 1,
       "terrain_materials": 4,
       "voxel_relief_m": 19,
-      "micro_habitat_distinct": 0
+      "micro_habitat_distinct": 1
     },
     "coherence": {
       "water_districts": 25,
@@ -78,7 +78,7 @@
       "drainage_samples": 0,
       "drainage_monotonic": 0,
       "vegetation_samples": 64,
-      "vegetated_districts": 0
+      "vegetated_districts": 39
     }
   }
 ]