Limited borrowed types to modded

- Pack validation rejects borrowedDimensionType on platforms that generate their own Iris dimension type, with an error naming the supported loaders
- Dimension type keys and registry contracts honor borrowedDimensionType only when the platform generation registry supports borrowed types, so Bukkit always derives its generated iris type
- The Bukkit runtime contract always expects the generated Iris type; hotload still refuses a borrowedDimensionType change

core/src/main/java/art/arcane/iris/generation/terrain/IrisDimension.java

@@ -1112,8 +1112,12 @@ public class IrisDimension extends IrisRegistrant {
         return borrowedDimensionType != null && !borrowedDimensionType.isBlank();
     }
 
+    public boolean borrowsDimensionType(PlatformGenerationRegistry registry) {
+        return hasBorrowedDimensionType() && registry.supportsBorrowedDimensionTypes();
+    }
+
     public String dimensionTypeResourceKey(PlatformGenerationRegistry registry, String packName) {
-        if (hasBorrowedDimensionType()) {
+        if (borrowsDimensionType(registry)) {
             return borrowedDimensionType.trim();
         }
         return registry.dimensionTypeResourceKey(packName, getLoadKey(), getDimensionTypeKey());

core/src/main/java/art/arcane/iris/generation/terrain/IrisDimensionRuntimeContract.java

@@ -46,9 +46,7 @@ public record IrisDimensionRuntimeContract(
         Objects.requireNonNull(dimension, "dimension");
         Objects.requireNonNull(namespace, "namespace");
         return new IrisDimensionRuntimeContract(
-                dimension.hasBorrowedDimensionType()
-                        ? dimension.getBorrowedDimensionType().trim()
-                        : namespace + ":" + dimension.getDimensionTypeKey(),
+                namespace + ":" + dimension.getDimensionTypeKey(),
                 dimension.getMinHeight(),
                 dimension.getMaxHeight() - dimension.getMinHeight(),
                 dimension.getLogicalHeight());
@@ -63,6 +61,7 @@ public record IrisDimensionRuntimeContract(
         IrisDimensionRuntimeContract actual = expected(active, namespace);
         IrisDimensionRuntimeContract proposed = expected(replacement, namespace);
         proposed.requireExact(runtimeName, actual);
+        requireBorrowedTypeUnchanged(runtimeName, active, replacement);
         requireGeneratedTypeCompatible(runtimeName, active, replacement);
     }
 
@@ -120,6 +119,25 @@ public record IrisDimensionRuntimeContract(
                 + ". Generation was refused before any chunk writes. Restart after installing the exact Iris dimension type; terrain clipping is not allowed.");
     }
 
+    private static void requireBorrowedTypeUnchanged(
+            String runtimeName,
+            IrisDimension active,
+            IrisDimension replacement
+    ) {
+        String activeBorrowed = borrowedType(active);
+        String replacementBorrowed = borrowedType(replacement);
+        if (activeBorrowed.equals(replacementBorrowed)) {
+            return;
+        }
+        throw new IrisDimensionContractException(runtimeName + " cannot hotload a changed borrowedDimensionType. The active pack borrows '"
+                + activeBorrowed + "' and the replacement borrows '" + replacementBorrowed
+                + "'. Close and reopen Studio after changing borrowedDimensionType.");
+    }
+
+    private static String borrowedType(IrisDimension dimension) {
+        return dimension.hasBorrowedDimensionType() ? dimension.getBorrowedDimensionType().trim() : "";
+    }
+
     private static void requireGeneratedTypeCompatible(
             String runtimeName,
             IrisDimension active,

core/src/main/java/art/arcane/iris/pack/PackDimensionValidator.java

@@ -19,6 +19,7 @@
 package art.arcane.iris.pack;
 
 import art.arcane.iris.generation.terrain.IrisDimensionType;
+import art.arcane.iris.spi.IrisPlatforms;
 import art.arcane.iris.structure.object.IrisObjectScale;
 import art.arcane.iris.world.IrisWorldBoundary;
 import art.arcane.volmlib.util.json.JSONArray;
@@ -60,7 +61,7 @@ final class PackDimensionValidator {
 
             validateImportedStructurePolicy(dimensionKey, dimJson, blockingErrors, warnings);
             validateDimensionHeights(packFolder, dimensionKey, dimJson, blockingErrors);
-            validateBorrowedDimensionType(dimensionKey, dimJson, blockingErrors);
+            validateBorrowedDimensionType(dimensionKey, dimJson, borrowedDimensionTypesSupported(), blockingErrors);
             validateSamplingStep(dimensionKey, dimJson, "terrainSamplingStep", 1, blockingErrors);
             validateSamplingStep(dimensionKey, dimJson, "caveDensitySamplingStep", 1, blockingErrors);
             validateSamplingStep(dimensionKey, dimJson, "biomeBoundsSamplingStep", 4, blockingErrors);
@@ -128,16 +129,29 @@ final class PackDimensionValidator {
         }
     }
 
-    static void validateBorrowedDimensionType(String dimensionKey, JSONObject dimension, List<String> errors) {
+    static void validateBorrowedDimensionType(String dimensionKey, JSONObject dimension, boolean borrowingSupported,
+                                              List<String> errors) {
         if (!dimension.has(BORROWED_DIMENSION_TYPE) || dimension.isNull(BORROWED_DIMENSION_TYPE)) {
             return;
         }
         Object value = dimension.opt(BORROWED_DIMENSION_TYPE);
-        if (value instanceof String key && (key.isEmpty() || RESOURCE_KEY.matcher(key).matches())) {
+        if (value instanceof String key && key.isEmpty()) {
             return;
         }
-        errors.add("Dimension '" + dimensionKey + "' " + BORROWED_DIMENSION_TYPE
-                + " must be a namespaced dimension type key such as aether:the_aether, but was '" + value + "'.");
+        if (!(value instanceof String key) || !RESOURCE_KEY.matcher(key).matches()) {
+            errors.add("Dimension '" + dimensionKey + "' " + BORROWED_DIMENSION_TYPE
+                    + " must be a namespaced dimension type key such as aether:the_aether, but was '" + value + "'.");
+            return;
+        }
+        if (!borrowingSupported) {
+            errors.add("Dimension '" + dimensionKey + "' sets " + BORROWED_DIMENSION_TYPE + " to '" + key
+                    + "', which only Fabric, Forge, and NeoForge servers support. This server generates its own Iris dimension type, so remove "
+                    + BORROWED_DIMENSION_TYPE + " to use this pack here.");
+        }
+    }
+
+    private static boolean borrowedDimensionTypesSupported() {
+        return !IrisPlatforms.isBound() || IrisPlatforms.get().supportsBorrowedDimensionTypes();
     }
 
     static void validateObjectScaleFactor(String dimensionKey, JSONObject dimension, List<String> errors) {

core/src/main/java/art/arcane/iris/world/history/GenerationRegistryContractFactory.java

@@ -175,7 +175,7 @@ public final class GenerationRegistryContractFactory {
         List<IrisBiome> reachableBiomes = new ArrayList<>(requiredDimension.getReachableBiomes(() -> requiredData));
         for (IrisDimension registryDimension : registryDimensions(requiredData, requiredDimension)) {
             String dimensionKey = requireText(registryDimension.getLoadKey(), "Dimension load key");
-            if (!registryDimension.hasBorrowedDimensionType()) {
+            if (!registryDimension.borrowsDimensionType(requiredGenerationRegistry)) {
                 addGeneratedDimensionType(
                         definitions,
                         generatedSources,