Enforce seed cohesion

- Scope cached generation fields to their seed and context.
- Warm decorators and ores with their canonical generation seeds.
- Stabilize object traversal and procedural placement choices.
- Isolate smart-bore variants from shared source objects.
- Pin noise and full-engine output across generation orders.
- Verify same-jar generation through cold server restarts.

probe/src/test/resources/seed-cohesion/regions/terrain.json

@@ -0,0 +1,6 @@
+{
+  "name": "Terrain",
+  "landBiomes": ["terrain"],
+  "seaBiomes": ["terrain"],
+  "shoreBiomes": ["terrain"]
+}