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;

core/src/test/java/art/arcane/iris/generation/runtime/PreparedHydrologyCacheIdentityTest.java

@@ -1,6 +1,7 @@
 package art.arcane.iris.generation.runtime;
 
 import art.arcane.iris.world.history.GenerationKernelRegistry;
+import art.arcane.iris.world.history.GenerationPackFingerprint;
 import art.arcane.iris.world.history.TransitionGenerationPlan;
 import org.junit.Test;
 import org.junit.Rule;
@@ -21,6 +22,7 @@ import static org.mockito.Mockito.mockStatic;
 
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertNotEquals;
+import static org.mockito.Mockito.doReturn;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.when;
 
@@ -121,6 +123,8 @@ public class PreparedHydrologyCacheIdentityTest {
         IrisData data = mock(IrisData.class);
         when(data.getDataFolder()).thenReturn(path.toFile());
         when(data.hasGenerationRegistryContract()).thenReturn(true);
+        when(data.generationPackFingerprint()).thenAnswer(invocation ->
+                GenerationPackFingerprint.compute(path, GenerationPackFingerprint.CURRENT_VERSION));
         IrisWorld world = mock(IrisWorld.class);
         when(world.minHeight()).thenReturn(-64);
         when(world.maxHeight()).thenReturn(320);
@@ -133,6 +137,23 @@ public class PreparedHydrologyCacheIdentityTest {
         return target;
     }
 
+    @Test
+    public void immutableGenerationPackUsesTheDataFingerprintWithoutRereadingThePack() throws Exception {
+        Path missing = folder.getRoot().toPath().resolve("immutable-epoch-pack");
+        EngineTarget target = target(missing);
+        IrisData data = target.getData();
+        doReturn("verified-epoch").when(data).generationPackFingerprint();
+        GenerationKernelRegistry.RuntimeKernel kernel = kernel("implementation");
+        try (MockedStatic<PackValidationCache> validation = mockStatic(PackValidationCache.class)) {
+            validation.when(PackValidationCache::contextFingerprint).thenReturn("platform");
+
+            PreparedHydrologyCacheIdentity captured = PreparedHydrologyCacheIdentity.capture(target, kernel, null, false);
+
+            assertEquals(PreparedHydrologyCacheIdentity.fromInputs(
+                    "verified-epoch", "closed", "platform", kernel, null, -64, 320, 0), captured);
+        }
+    }
+
     @Test
     public void identicalInputsShareRegardlessOfWorldLocation() {
         GenerationKernelRegistry.RuntimeKernel kernel = kernel("implementation");