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/data/mosaic_constants.toml

@@ -107,38 +107,160 @@ water = "Dry"
 weight = 15
 affinity = "any"
 
-[[grassland]]
-name = "tussock"
+# Grassland is BIOME-KEYED (T-1085, D-246): the ClimateBiome derived from the district's
+# temperature_c + moisture_q selects savanna / temperate / boreal / tundra, so a
+# grassland reads distinctly per climate rather than as one biome-blind palette. §8-safe
+# (vegetation + water; terrain stays Soil in v1). Names per
+# docs/design/alien-ecology-vocabulary.md.
+
+[[grassland_temperate]]
+name = "cane-tassel sweep"
 terrain = "Soil"
 vegetation = "Grass"
 water = "Dry"
 weight = 40
 affinity = "any"
 
-[[grassland]]
-name = "scrub island"
+[[grassland_temperate]]
+name = "drum-root knot"
 terrain = "Soil"
 vegetation = "Scrub"
 water = "Dry"
 weight = 18
 affinity = "dry"
 
-[[grassland]]
-name = "wildflower"
+[[grassland_temperate]]
+name = "wrack-bloom"
 terrain = "Soil"
 vegetation = "Meadow"
 water = "Dry"
 weight = 17
 affinity = "dry"
 
-[[grassland]]
-name = "dry wash"
-terrain = "Gravel"
+[[grassland_temperate]]
+name = "husk-line seep"
+terrain = "Soil"
 vegetation = "Barren"
 water = "Dry"
 weight = 10
 affinity = "wet"
 
+# Savanna: scattered Forest copses in a grass matrix — the vane-crown copse is the
+# clearest distinction from temperate grassland.
+[[grassland_savanna]]
+name = "cane-tassel sweep"
+terrain = "Soil"
+vegetation = "Grass"
+water = "Dry"
+weight = 38
+affinity = "any"
+
+[[grassland_savanna]]
+name = "vane-crown copse"
+terrain = "Soil"
+vegetation = "Forest"
+water = "Dry"
+weight = 12
+affinity = "dry"
+
+[[grassland_savanna]]
+name = "husk-thorn scrub"
+terrain = "Soil"
+vegetation = "Scrub"
+water = "Dry"
+weight = 18
+affinity = "dry"
+
+[[grassland_savanna]]
+name = "husk-pan waterhole"
+terrain = "Soil"
+vegetation = "Grass"
+water = "Shallow"
+weight = 10
+affinity = "wet"
+
+[[grassland_savanna]]
+name = "drum-mound flat"
+terrain = "Soil"
+vegetation = "Barren"
+water = "Dry"
+weight = 12
+affinity = "dry"
+
+# Boreal grassland: cooler, sparser — more scrub, a lichen fraction, no meadow.
+[[grassland_boreal]]
+name = "cane-tassel sweep"
+terrain = "Soil"
+vegetation = "Grass"
+water = "Dry"
+weight = 30
+affinity = "any"
+
+[[grassland_boreal]]
+name = "wrack-scrub"
+terrain = "Soil"
+vegetation = "Scrub"
+water = "Dry"
+weight = 25
+affinity = "dry"
+
+[[grassland_boreal]]
+name = "husk-line seep"
+terrain = "Soil"
+vegetation = "Grass"
+water = "Shallow"
+weight = 10
+affinity = "wet"
+
+[[grassland_boreal]]
+name = "wrack-lichen mat"
+terrain = "Soil"
+vegetation = "Lichen"
+water = "Dry"
+weight = 12
+affinity = "any"
+
+[[grassland_boreal]]
+name = "cold scald"
+terrain = "Soil"
+vegetation = "Barren"
+water = "Dry"
+weight = 15
+affinity = "dry"
+
+# Tundra: sparse and cold — scrub, lichen, bare patterned ground, NO forest.
+[[grassland_tundra]]
+name = "husk-heath brake"
+terrain = "Soil"
+vegetation = "Scrub"
+water = "Dry"
+weight = 26
+affinity = "any"
+
+[[grassland_tundra]]
+name = "wrack-lichen mat"
+terrain = "Soil"
+vegetation = "Lichen"
+water = "Dry"
+weight = 24
+affinity = "any"
+
+[[grassland_tundra]]
+name = "thermokarst husk-pool"
+terrain = "Soil"
+vegetation = "Grass"
+water = "Shallow"
+weight = 10
+affinity = "wet"
+
+[[grassland_tundra]]
+name = "lichen-scald flat"
+terrain = "Soil"
+vegetation = "Barren"
+water = "Dry"
+weight = 30
+affinity = "dry"
+
 # ── Follow-up (§8-aware material pass): Sand / Rock / Gravel / Lava ──────────────
 
 [[sand]]