feat(simulation): Layer-1 topography pipeline — features, sub-biome, orchestrator (#953)

Wire the empty-world topography cascade (D-208/209/210):
- generator.rs: add SubBiomeVariant (11 variants, D-210) + sub_biome and
  terrain_modification_cost fields on GeographicAttractor; AttractorType
  is now Copy.
- features.rs (new, D-209): extract the 7 attractor tags from heightmap +
  drainage. Coast/lake derived from the heightmap (D-209/D-223
  reconciliation — markers are names-only now, no polygons). Deterministic
  (sorted seeds, integer keys, bucket-grid thinning); strength-capped at
  MAX_ATTRACTORS preserving type diversity. Shared TerrainAnalysis
  (masks/slope/moisture/percentile) feeds both extraction and sub-biome.
- subbiome.rs (new, D-210): classify sub-biome + terrain_modification_cost
  from elevation/slope/moisture/latitude. Volcanic stays in the enum but
  is not emitted (no heightmap signal).
- layer1.rs (new): run_layer1 orchestrator + attach_feature_names (D-223
  pool names to largest rivers / Alpine peaks).
- attractor_matching constructors updated for the new fields.

76 atlas tests pass; run_layer1 determinism verified.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

server/src/atlas/mod.rs

@@ -8,8 +8,11 @@ pub mod block_irregularity;
 pub mod body_world_state;
 pub mod district_mix;
 pub mod drainage;
+pub mod features;
 pub mod gen_queue;
 pub mod heightmap;
+pub mod layer1;
 pub mod rng;
 pub mod skeleton_gen;
+pub mod subbiome;
 pub mod tile_condition;