Fixed shutdown teardown and shutdown staging wait

- Server stop defers teardown only while the plugin class loader close is held
- Shutdown with no Iris world loaded tears down during plugin disable
- Pre-unload hook drains generators inline when the class loader close cannot be held
- INMSBinding reports whether the class loader close is held
- Shutdown tests cover the held and unheld pre-unload paths
- Shutdown waits for an in-progress world replacement to finish staging
- Replacements started after shutdown begins are refused
- Admission tests cover the shutdown wait and the refusal

core/src/main/java/art/arcane/iris/platform/bukkit/nms/INMSBinding.java

@@ -301,7 +301,8 @@ public interface INMSBinding extends NativeTerrainAccess {
     default void uninjectBukkit() {
     }
 
-    default void deferPluginClassLoaderClose() {
+    default boolean deferPluginClassLoaderClose() {
+        return false;
     }
 
     default void releasePluginClassLoaderClose() {

core/src/test/java/art/arcane/iris/platform/bukkit/nms/BukkitBindingTest.java

@@ -30,7 +30,8 @@ public class BukkitBindingTest {
         Fixture fixture = new Fixture();
         when(fixture.lifecycle.isServerStopping()).thenReturn(true);
         assertTrue(fixture.binding.isServerStopping());
-        fixture.binding.deferPluginClassLoaderClose();
+        when(fixture.lifecycle.deferPluginClassLoaderClose()).thenReturn(true);
+        assertTrue(fixture.binding.deferPluginClassLoaderClose());
         fixture.binding.releasePluginClassLoaderClose();
         verify(fixture.lifecycle).deferPluginClassLoaderClose();
         verify(fixture.lifecycle).releasePluginClassLoaderClose();