Drop stale cave snapshots

- dimension stack metadata clearing and continuation restore rewrite cavern cells after sCave, so they drop the context snapshot and decoration re-reads the mantle
- test that decoration reuses only its own chunk's snapshot

core/src/main/java/art/arcane/iris/generation/mantle/CaveTerrainSnapshot.java

@@ -17,8 +17,9 @@ import java.util.function.IntFunction;
 /**
  * The terrain view of one mantle chunk's cavern and hydrology cells (journaled pre-object values win),
  * copied once under the chunk monitor. Terrain components finish before any content write reaches a
- * chunk and every later write journals the value it replaces, so this view never goes stale while the
- * chunk generates.
+ * chunk and every mantle-writer write journals the value it replaces, so the view holds while the chunk
+ * generates; the raw writers that run after sCave (dimension stack clearing, continuation restore) drop
+ * it from the chunk context instead.
  */
 public final class CaveTerrainSnapshot {
     private static final int SECTION_VOLUME = 4096;

core/src/main/java/art/arcane/iris/generation/runtime/EngineMode.java

@@ -218,6 +218,9 @@ public interface EngineMode extends Staged {
             IrisDimensionStackActuator.clearHostNaturalMetadata(getEngine(), x, z, biomes.getHeight(), ctx);
             ctx.beginContent();
             List<Matter> continuations = restoreObjectContinuations(x, z);
+            if (!continuations.isEmpty()) {
+                ctx.setCaveTerrain(null);
+            }
             for (EngineStage i : stages) {
                 i.generate(x, z, blocks, biomes, multicore, ctx);
             }

core/src/main/java/art/arcane/iris/generation/stage/IrisDimensionStackActuator.java

@@ -110,6 +110,7 @@ public final class IrisDimensionStackActuator extends EngineAssignedActuator<Nat
         if (!context.hasDimensionStack() || !context.getComplex().allowsMantleChunkWrite(x >> 4, z >> 4)) {
             return;
         }
+        context.setCaveTerrain(null);
         MantleChunk<Matter> chunk = engine.getMantle().getMantle().getChunk(x >> 4, z >> 4).use();
         try {
             synchronized (chunk) {