Preserved historical biome behavior

- Generated chunks retain surface and vertical biome assignments
- Floating biome assignments persist through terrain transitions
- Archived pack definitions remain available for saved biome rules
- Spawns and effects use each saved biome environment
- Saved biome lookups load asynchronously with bounded caches
- Missing biome identities recover only from exact recorded evidence
- Effects and entity placement follow existing world surfaces
- New pack fingerprints ignore Finder metadata
- Tests cover biome storage, recovery, consumers, and scoped definitions

adapters/bukkit/nms/v26_2_R1/src/main/java/art/arcane/iris/core/nms/v26_2_R1/CustomBiomeSource.java

@@ -333,6 +333,10 @@ public class CustomBiomeSource extends BiomeSource {
         return Collections.unmodifiableMap(spawnBiomes);
     }
 
+    Holder<Biome> getRetainedVanillaSpawnBiome(String derivativeKey) {
+        return resolveBiomeHolder(biomeRegistry, derivativeKey);
+    }
+
     Holder<Biome> getVanillaSpawnBiome(Holder<Biome> biome) {
         if (biome == null) {
             return null;

adapters/bukkit/nms/v26_2_R1/src/main/java/art/arcane/iris/core/nms/v26_2_R1/IrisChunkGenerator.java

@@ -18,6 +18,7 @@ import art.arcane.iris.engine.framework.NativeStructureGenerationPolicy;
 import art.arcane.iris.engine.IrisEngine;
 import art.arcane.iris.engine.history.GenerationHistoryRuntimeRouter;
 import art.arcane.iris.engine.history.NativeTerrainReceipt;
+import art.arcane.iris.engine.history.NativeBiomeSpawnSelection;
 import art.arcane.iris.engine.history.SavedTerrainChunk;
 import org.bukkit.NamespacedKey;
 import org.bukkit.persistence.PersistentDataType;
@@ -929,11 +930,17 @@ public class IrisChunkGenerator extends CustomChunkGenerator implements LongPred
 
     @Override
     public WeightedList<MobSpawnSettings.SpawnerData> getMobsAt(Holder<Biome> holder, StructureManager structuremanager, MobCategory enumcreaturetype, BlockPos blockposition) {
+        NativeBiomeSpawnSelection selection = NativeBiomeSpawnSelection.at(
+                engine, blockposition.getX(), blockposition.getY(), blockposition.getZ(),
+                holder.unwrapKey().map(key -> key.identifier().toString()).orElse(""));
+        if (selection.mode() == NativeBiomeSpawnSelection.Mode.LOADING) {
+            return WeightedList.of(List.of());
+        }
         try (GenerationHistoryRuntimeRouter.CoordinateScope route = openHistoryCoordinateScope(
                      blockposition.getX(), blockposition.getZ(), "bukkit_nms_mob_spawns");
              GenerationSessionLease lease = requireGenerationLease("bukkit_nms_mob_spawns");
              IrisContext.Scope ignored = IrisContext.open(engine, lease.sessionId(), null)) {
-            return getMobsAtWithActiveRuntime(holder, structuremanager, enumcreaturetype, blockposition);
+            return getMobsAtWithActiveRuntime(holder, structuremanager, enumcreaturetype, blockposition, selection);
         }
     }
 
@@ -941,9 +948,14 @@ public class IrisChunkGenerator extends CustomChunkGenerator implements LongPred
             Holder<Biome> holder,
             StructureManager structuremanager,
             MobCategory enumcreaturetype,
-            BlockPos blockposition
+            BlockPos blockposition,
+            NativeBiomeSpawnSelection selection
     ) {
-        Holder<Biome> vanillaSpawnBiome = customBiomeSource.getVanillaSpawnBiome(holder);
+        Holder<Biome> vanillaSpawnBiome = switch (selection.mode()) {
+            case RETAINED -> customBiomeSource.getRetainedVanillaSpawnBiome(selection.derivativeKey());
+            case CURRENT -> customBiomeSource.getVanillaSpawnBiome(holder);
+            case NONE, LOADING -> null;
+        };
         if (vanillaSpawnBiome == null) {
             return delegate.getMobsAt(holder, structuremanager, enumcreaturetype, blockposition);
         }
@@ -964,7 +976,7 @@ public class IrisChunkGenerator extends CustomChunkGenerator implements LongPred
         }
 
         int spawnRuntimeId = engine.getCacheID();
-        SpawnTableKey key = new SpawnTableKey(spawnRuntimeId, holder.value(), enumcreaturetype);
+        SpawnTableKey key = new SpawnTableKey(spawnRuntimeId, holder.value(), vanillaSpawnBiome.value(), enumcreaturetype);
         return mergedSpawnTables.computeIfAbsent(key, ignored -> mergeSpawnTables(vanillaSpawns, explicitSpawns));
     }
 
@@ -1611,7 +1623,7 @@ public class IrisChunkGenerator extends CustomChunkGenerator implements LongPred
         }
     }
 
-    private record SpawnTableKey(int runtimeId, Biome biome, MobCategory category) {
+    private record SpawnTableKey(int runtimeId, Biome biome, Biome vanillaBiome, MobCategory category) {
     }
 
     private record ReachableStructureCache(IrisDimension dimension, int runtimeId, Set<String> keys) {