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

spi/src/main/java/art/arcane/iris/spi/IrisPlatform.java

@@ -76,6 +76,10 @@ public interface IrisPlatform {
         return false;
     }
 
+    default boolean supportsBorrowedDimensionTypes() {
+        return false;
+    }
+
     /**
      * Root folder Iris owns for packs, settings and generated data. Created if missing. Never null.
      */

spi/src/main/java/art/arcane/iris/spi/PlatformGenerationRegistry.java

@@ -25,6 +25,10 @@ public interface PlatformGenerationRegistry extends NativeGenerationRegistry {
 
     String dimensionTypeResourceKey(String packName, String dimensionKey, String dimensionTypeKey);
 
+    default boolean supportsBorrowedDimensionTypes() {
+        return false;
+    }
+
     static String contentAddressedCustomBiomeResourceKey(String identitySha256) {
         String requiredFingerprint = Objects.requireNonNull(identitySha256, "identitySha256");
         if (!SHA_256_PATTERN.matcher(requiredFingerprint).matches()) {