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) {

core/src/test/java/art/arcane/iris/generation/stage/IrisNaturalCaveScanTest.java

@@ -5,6 +5,7 @@ import art.arcane.iris.generation.decoration.IrisCeilingDecorator;
 import art.arcane.iris.generation.decoration.IrisSurfaceDecorator;
 import art.arcane.iris.generation.runtime.Engine;
 import art.arcane.iris.generation.runtime.SeedManager;
+import art.arcane.iris.generation.context.ChunkContext;
 import art.arcane.iris.generation.mantle.CaveTerrainSnapshot;
 import art.arcane.iris.generation.mantle.EngineMantle;
 import art.arcane.iris.generation.biome.IrisBiome;
@@ -33,6 +34,7 @@ import java.util.ArrayList;
 import java.util.List;
 
 import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertSame;
 import static org.junit.Assert.assertThrows;
 import static org.junit.Assert.assertTrue;
@@ -42,10 +44,12 @@ import static org.mockito.ArgumentMatchers.anyString;
 import static org.mockito.ArgumentMatchers.argThat;
 import static org.mockito.ArgumentMatchers.eq;
 import static org.mockito.Mockito.CALLS_REAL_METHODS;
+import static org.mockito.Mockito.clearInvocations;
 import static org.mockito.Mockito.doAnswer;
 import static org.mockito.Mockito.doReturn;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.mockStatic;
+import static org.mockito.Mockito.never;
 import static org.mockito.Mockito.verify;
 
 public class IrisNaturalCaveScanTest {
@@ -72,6 +76,26 @@ public class IrisNaturalCaveScanTest {
         verifyScan(16, 16);
     }
 
+    @Test
+    public void decorationReusesOnlyTheCarveSnapshotOfItsOwnChunk() throws Exception {
+        Fixture fixture = new Fixture();
+        ChunkContext context = mock(ChunkContext.class);
+        doReturn(CaveTerrainSnapshot.capture(fixture.chunk, -2, 3)).when(context).getCaveTerrain();
+        clearInvocations(fixture.chunk);
+        fixture.modifier.decorateNaturalCaves(-32, 48, fixture.output(3, 2), context);
+        verify(fixture.chunk, never()).use();
+        verify(context).setCaveTerrain(null);
+        assertFalse(fixture.calls.isEmpty());
+
+        Fixture other = new Fixture();
+        ChunkContext stale = mock(ChunkContext.class);
+        doReturn(CaveTerrainSnapshot.capture(other.chunk, 5, 5)).when(stale).getCaveTerrain();
+        clearInvocations(other.chunk);
+        other.modifier.decorateNaturalCaves(-32, 48, other.output(3, 2), stale);
+        verify(other.chunk).use();
+        assertEquals(fixture.calls, other.calls);
+    }
+
     @Test
     public void listenerFailureReleasesMantleChunk() throws Exception {
         Fixture fixture = new Fixture();