Spread native volume cache keys

- Mix chunk keys so origin and query caches stop treeifying buckets

core/src/test/java/art/arcane/iris/structure/nativegen/NativeStructureVolumeIndexKeyTest.java

@@ -0,0 +1,23 @@
+package art.arcane.iris.structure.nativegen;
+
+import it.unimi.dsi.fastutil.ints.IntOpenHashSet;
+import org.junit.Test;
+
+import static org.junit.Assert.assertTrue;
+
+public class NativeStructureVolumeIndexKeyTest {
+    @Test
+    public void neighbouringChunkKeysSpreadAcrossHashBuckets() {
+        IntOpenHashSet buckets = new IntOpenHashSet();
+        int keys = 0;
+        for (int chunkX = -64; chunkX < 64; chunkX++) {
+            for (int chunkZ = -64; chunkZ < 64; chunkZ++) {
+                long chunkKey = ((long) chunkX << 32) ^ (chunkZ & 0xffffffffL);
+                int hash = NativeStructureVolumeIndex.runtimeChunkHash(7, chunkKey);
+                buckets.add((hash ^ (hash >>> 16)) & ((1 << 15) - 1));
+                keys++;
+            }
+        }
+        assertTrue("buckets=" + buckets.size(), buckets.size() > keys / 2);
+    }
+}