Restored saved dimensions off-thread

- Saved Iris dimensions are prepared in parallel during world load (server-about-to-start on NeoForge and Forge, server-starting on Fabric), before the server watchdog runs
- Server start only creates the restored levels, builds their engines in parallel, and publishes them, so its wait no longer grows one world at a time
- Each restored dimension opens its generation history once instead of twice and resolves its active pack once instead of three times
- An epoch snapshot whose content matches its validated source pack reuses that validation instead of running the full pack validator for every world
- Level creation reads the configured pack and structure biome keys from data warmed during preparation
- Immutable generation packs feed the prepared hydrology cache identity from the data's memoized fingerprint instead of hashing the pack twice per engine build
- Restore logging reports each dimension's preparation and engine time and how long the server thread waited

core/src/main/java/art/arcane/iris/generation/runtime/PreparedHydrologyCacheIdentity.java

@@ -30,9 +30,12 @@ record PreparedHydrologyCacheIdentity(String fingerprint) {
             if (context.isBlank()) {
                 return null;
             }
-            String content = GenerationPackFingerprint.compute(
-                    target.getData().getDataFolder().toPath(), GenerationPackFingerprint.CURRENT_VERSION);
-            String dependencies = target.getData().hasGenerationRegistryContract() ? "closed"
+            boolean immutable = target.getData().hasGenerationRegistryContract();
+            String content = immutable
+                    ? target.getData().generationPackFingerprint()
+                    : GenerationPackFingerprint.compute(
+                            target.getData().getDataFolder().toPath(), GenerationPackFingerprint.CURRENT_VERSION);
+            String dependencies = immutable ? "closed"
                     : dependencyFingerprint(IrisPlatforms.get().packsFolderNoCreate());
             if (dependencies.isBlank()) {
                 return null;