feat(simulation): integer-deterministic Layer-3 settlement placement (#955)

Wire the existing attractor-matching engine (#919/#925) into the
generation cascade as Layer 3, and make the whole placement-scoring path
integer-deterministic.

Layer 3 (D-211):
- CascadeLayer::Settlement + Layer3Output (placements) on the snapshot;
  BodyWorldState gains a `placements` field (the D-203 hot cache).
- run_layer3 runs the five-phase match_cities against Layer-1 attractors
  via the authored D-195 compatibility matrix; pure function of
  (attractors, cities) — no RNG. cities are passed in by the caller so the
  cascade stays DB-free and testable. A `// cache seam` marks where a
  persistent cache wraps it later (#1021).
- gen_queue passes &[] for now (Topography needs no cities); the runtime
  settlement read (gen_queue/layer_proxy) is the #955 follow-on.

Integer determinism (D-010 / D-227 — D-195 amended):
- Wiring match_cities into the deterministic cascade made its f32 scoring
  a live cross-platform divergence risk (a near-tie comparison or the
  Hungarian's f32 reductions can round differently per platform → a
  different world from the same seed). Converted the entire path to
  integers: CompatibilityMatrix is a 0-100 affinity table; attractor
  strength is 0-100 and terrain cost is a percent (100 = baseline),
  quantized once at the Layer-1 feature boundary; cell_score, the
  Hungarian, and CityPlacement.score are i64. No f32 in any placement or
  ranking decision.
- Layer-1 golden fixture rebaked: confirmed selection/positions are
  unchanged (same 256 attractors, 93 river cells) — only the strength/cost
  representation changed.

Tests: lib green (1292); new settlement_layer_places_cities_deterministically
covers placement + determinism + propagation into BodyWorldState.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

server/tests/cascade_golden.rs

@@ -55,7 +55,7 @@ fn cascade_layer0_to_1_matches_golden() {
     // ── Layer 1 — downsample, then run the cascade to topography. ───────────
     let small = heightmap.downsample(DOWNSAMPLE.0, DOWNSAMPLE.1);
     let body_seed = SeedChain::for_body(WORLD_SEED, "GJ1c");
-    let snapshot = run_cascade_from_heightmap(body_seed, small, CascadeLayer::Topography);
+    let snapshot = run_cascade_from_heightmap(body_seed, small, &[], CascadeLayer::Topography);
     let layer1 = snapshot.layer1.expect("Layer 1 ran");
 
     let actual = json!({

server/tests/gen_fixtures.rs

@@ -557,9 +557,9 @@ fn generate_atlas_layer_response_fixtures() {
         attractors: vec![GeographicAttractor {
             position: (12, 58),
             attractor_type: AttractorType::CoastalAccess,
-            strength: 0.9,
+            strength: 90,
             sub_biome: SubBiomeVariant::CoastalLowland,
-            terrain_modification_cost: 1.7,
+            terrain_modification_cost: 170,
         }],
         grid_w: 512,
         grid_h: 256,