Column biome fallback

- Column biome hunk takes its fallback biome from the caller
- The fallback resolves on first unset read
- Bukkit chunk generator supplies plains
- Core keeps no new Bukkit coupling

core/src/main/java/art/arcane/iris/generation/chunk/ColumnBiomeHunk.java

@@ -18,26 +18,29 @@
 
 package art.arcane.iris.generation.chunk;
 
-import art.arcane.iris.platform.bukkit.BukkitBiome;
 import art.arcane.volmlib.nativelib.terrain.NativeBiome;
 import art.arcane.volmlib.util.hunk.HunkMutationSupport;
 import art.arcane.volmlib.util.hunk.storage.StorageHunk;
-import org.bukkit.block.Biome;
+
+import java.util.function.Supplier;
 
 /**
  * Biome output for one Bukkit chunk, read back only by the generation stages. The biome actuator writes whole
  * columns, so storage stays at one entry per column until a partial column write arrives; that write switches
- * the hunk to full resolution. Reads, clamping and the plains fallback match {@link LinkedTerrainChunk}.
+ * the hunk to full resolution. Reads and clamping match {@link LinkedTerrainChunk}; unset cells read the
+ * caller's fallback biome, resolved on first use.
  */
 public final class ColumnBiomeHunk extends StorageHunk<NativeBiome> {
     private static final int CHUNK_SIZE = 16;
 
     private final NativeBiome[] columns = new NativeBiome[CHUNK_SIZE * CHUNK_SIZE];
     private NativeBiome[] cells;
+    private final Supplier<NativeBiome> fallback;
     private volatile NativeBiome defaultBiome;
 
-    public ColumnBiomeHunk(int height) {
+    public ColumnBiomeHunk(int height, Supplier<NativeBiome> fallback) {
         super(CHUNK_SIZE, height, CHUNK_SIZE);
+        this.fallback = fallback;
     }
 
     @Override
@@ -63,12 +66,12 @@ public final class ColumnBiomeHunk extends StorageHunk<NativeBiome> {
         if (biome != null) {
             return biome;
         }
-        NativeBiome fallback = defaultBiome;
+        NativeBiome resolved = defaultBiome;
-        if (fallback == null) {
+        if (resolved == null) {
-            fallback = BukkitBiome.of(Biome.PLAINS);
+            resolved = fallback.get();
-            defaultBiome = fallback;
+            defaultBiome = resolved;
         }
-        return fallback;
+        return resolved;
     }
 
     private void fillColumn(int x, int z, NativeBiome biome) {

core/src/main/java/art/arcane/iris/platform/generation/BukkitChunkGenerator.java

@@ -74,6 +74,7 @@ import art.arcane.volmlib.util.collection.KList;
 import art.arcane.volmlib.util.math.M;
 import art.arcane.volmlib.util.hunk.Hunk;
 import art.arcane.iris.generation.chunk.ColumnBiomeHunk;
+import art.arcane.iris.platform.bukkit.BukkitBiome;
 import art.arcane.iris.generation.chunk.ChunkDataHunkHolder;
 import art.arcane.iris.generation.context.ChunkContext;
 import art.arcane.iris.generation.context.IrisContext;
@@ -90,6 +91,7 @@ import org.bukkit.event.EventHandler;
 import org.bukkit.event.EventPriority;
 import org.bukkit.event.Listener;
 import org.bukkit.event.world.WorldInitEvent;
+import org.bukkit.block.Biome;
 import org.bukkit.generator.BiomeProvider;
 import org.bukkit.generator.BlockPopulator;
 import org.bukkit.generator.ChunkGenerator;
@@ -1312,7 +1314,7 @@ public class BukkitChunkGenerator extends ChunkGenerator implements PlatformChun
                 selected.generateChunk(engine, TerrainChunk.create(d), x, z);
             } else {
                 ChunkDataHunkHolder blocks = new ChunkDataHunkHolder(d);
-                Hunk<NativeBiome> biomes = new ColumnBiomeHunk(d.getMaxHeight() - d.getMinHeight());
+                Hunk<NativeBiome> biomes = new ColumnBiomeHunk(d.getMaxHeight() - d.getMinHeight(), () -> BukkitBiome.of(Biome.PLAINS));
                 try (GenerationHistoryRuntimeRouter.CoordinateScope historyScope =
                              openGenerationHistoryCoordinateScope(engine, x << 4, z << 4);
                      GenerationSessionLease lease = engine.acquireGenerationLease("bukkit_terrain_stage");

core/src/test/java/art/arcane/iris/generation/chunk/LinkedTerrainChunkTest.java

@@ -121,7 +121,7 @@ public class LinkedTerrainChunkTest {
         for (int trial = 0; trial < 12; trial++) {
             Random random = new Random(9127L + trial);
             TerrainChunkBiomeHunkView expected = new TerrainChunkBiomeHunkView(chunk());
-            ColumnBiomeHunk actual = new ColumnBiomeHunk(16);
+            ColumnBiomeHunk actual = new ColumnBiomeHunk(16, () -> plains);
             int operations = 1 + random.nextInt(80);
             for (int operation = 0; operation < operations; operation++) {
                 NativeBiome biome = palette[random.nextInt(palette.length)];