Added dimension stacking

- Dimension files define upright top-to-bottom terrain layers
- Spacer values set exact air gaps between layers
- Optional noise styles shape blended layer boundaries
- Runtime queries resolve stacked terrain and biome ownership
- Pack validation rejects invalid stack definitions
- Project copy and hotload paths include stack dependencies
- Superflat fixtures cover gaps, blending, clipping, and fluids
- Platform tests cover stacked world generation contracts

adapters/bukkit/plugin/src/main/java/art/arcane/iris/core/runtime/BukkitEnginePlatformHooks.java

@@ -84,6 +84,11 @@ public final class BukkitEnginePlatformHooks implements EnginePlatformHooks {
                 "iris");
     }
 
+    @Override
+    public void prepareRuntimeHotload(Engine engine) {
+        INMS.get().invalidateNativeStructureVolumeIndex(engine);
+    }
+
     @Override
     public void applyWorldBoundary(Engine engine) {
         IrisDimension expectedDimension = engine.getDimension();