Reused cave density columns

- Cave passes reuse validated lattice columns in local scratch
- Generation releases retained columns after each carve
- Regression tests cover density parity and cache lifetime

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

@@ -40,6 +40,7 @@ final class CaveCarveScratch {
     final double[] planeThresholdLimit = new double[256];
     final boolean[] planeCarve = new boolean[256];
     final double[] adaptivePlaneDensity = new double[81];
+    final CaveDensityGrid.Column[] densityColumns = new CaveDensityGrid.Column[16];
     double[] verticalDensityLower = new double[9];
     double[] verticalDensityUpper = new double[9];
     int verticalDensityLowerY = Integer.MIN_VALUE;
@@ -79,8 +80,9 @@ final class CaveCarveScratch {
     int adaptiveGeometryAxisCells = -1;
     boolean fullWeightsInitialized;
 
-    void releaseSections() {
+    void releaseReferences() {
         Arrays.fill(sectionMatter, null);
         Arrays.fill(sectionSlices, null);
+        Arrays.fill(densityColumns, null);
     }
 }

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

@@ -37,7 +37,7 @@ final class CaveDensityGrid {
         if (cached == null) {
             cached = columns.putIfAbsent(key, new Column(owner, latticeX, latticeZ, height));
         }
-        return cached.owner == owner && cached.latticeX == latticeX && cached.latticeZ == latticeZ ? cached : null;
+        return cached.matches(owner, latticeX, latticeZ) ? cached : null;
     }
 
     void clear() {
@@ -57,6 +57,10 @@ final class CaveDensityGrid {
             segments = new AtomicReferenceArray<>(Math.ceilDiv(Math.max(1, height), SEGMENT_SIZE));
         }
 
+        boolean matches(Object owner, int latticeX, int latticeZ) {
+            return this.owner == owner && this.latticeX == latticeX && this.latticeZ == latticeZ;
+        }
+
         double get(int y) {
             int segment = y >> SEGMENT_SHIFT;
             if (segment >= segments.length()) {

core/src/main/java/art/arcane/iris/generation/mantle/IrisCaveCarver3D.java

@@ -449,7 +449,7 @@ public class IrisCaveCarver3D {
 
             return carved;
         } finally {
-            scratchCache.get().releaseSections();
+            scratchCache.get().releaseReferences();
             carving.set(nested);
             if (nested) {
                 scratchCache.set(previousScratch);
@@ -2466,8 +2466,19 @@ public class IrisCaveCarver3D {
 
     private double latticeDensity(CaveCarveScratch scratch, int x, int y, int z) {
         int latticeMask = (1 << CaveDensityGrid.LATTICE_SHIFT) - 1;
-        CaveDensityGrid.Column column = ((x | z) & latticeMask) != 0 ? null : densityGrid.column(this, densityGridOwner,
-                densityMaximumY + 1, x >> CaveDensityGrid.LATTICE_SHIFT, z >> CaveDensityGrid.LATTICE_SHIFT);
+        if (((x | z) & latticeMask) != 0) {
+            return samplePlaneDensity(scratch, x, y, z);
+        }
+        int latticeX = x >> CaveDensityGrid.LATTICE_SHIFT;
+        int latticeZ = z >> CaveDensityGrid.LATTICE_SHIFT;
+        int slot = ((latticeX & 3) << 2) | (latticeZ & 3);
+        CaveDensityGrid.Column column = parallelDensity ? scratch.densityColumns[slot] : null;
+        if (column == null || !column.matches(this, latticeX, latticeZ)) {
+            column = densityGrid.column(this, densityGridOwner, densityMaximumY + 1, latticeX, latticeZ);
+            if (parallelDensity) {
+                scratch.densityColumns[slot] = column;
+            }
+        }
         if (column == null) {
             return samplePlaneDensity(scratch, x, y, z);
         }