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/simulation/generator.rs

@@ -371,7 +371,7 @@ pub enum TerritorialStatus {
 
 /// The type of terrain feature that attracts settlement placement.
 /// Source: D-195, D-209
-#[derive(Serialize, Deserialize, Clone, Debug, PartialEq, Eq)]
+#[derive(Serialize, Deserialize, Clone, Copy, Debug, PartialEq, Eq)]
 pub enum AttractorType {
     /// Where a river meets sea level or coastline. Historically high-value.
     RiverMouth,
@@ -389,8 +389,29 @@ pub enum AttractorType {
     PlainCenter,
 }
 
+/// Fine-grained terrain classification carried by each `GeographicAttractor`.
+/// Classifies the local terrain more finely than the top-level `SettingType`;
+/// drives ZonePalette modifier selection (D-101) and `terrain_modification_cost`.
+/// Source: D-210
+#[derive(Serialize, Deserialize, Clone, Copy, Debug, PartialEq, Eq)]
+pub enum SubBiomeVariant {
+    TropicalWet,
+    TemperateForest,
+    TemperateGrassland,
+    BorealForest,
+    Tundra,
+    Desert,
+    Savanna,
+    Alpine,
+    Wetland,
+    CoastalLowland,
+    /// No heightmap-derivable signal in the Layer-1 inputs (elevation, slope,
+    /// moisture, latitude); reserved for a future volcanic data source (D-210).
+    Volcanic,
+}
+
 /// A terrain feature at a specific map position that influences city placement scoring.
-/// Source: D-195, D-209
+/// Source: D-195, D-209, D-210
 #[derive(Serialize, Deserialize, Clone, Debug)]
 pub struct GeographicAttractor {
     /// Pixel position in heightmap space [row, col].
@@ -398,6 +419,12 @@ pub struct GeographicAttractor {
     pub attractor_type: AttractorType,
     /// Normalized strength 0.0–1.0. Derived from flow accumulation or habitability score.
     pub strength: f32,
+    /// Fine-grained terrain classification at this position (D-210).
+    pub sub_biome: SubBiomeVariant,
+    /// Infrastructure-build cost multiplier (1.0 = baseline grassland; higher =
+    /// more expensive). Derived from `sub_biome` + local slope. Consumed by the
+    /// attractor-matching pipeline (D-211) to penalize marginal cities. (D-210)
+    pub terrain_modification_cost: f32,
 }
 
 /// Compatibility weights between economic roles and attractor types.