Implement short-lived stellar event handling and optimize batch processing; add tests for new functionality

tests/test_async_save.gd

@@ -92,6 +92,22 @@ func run() -> void:
     check(reader.load_data("shutdown", "test-password").state.energy == 6.0, "Teardown drains final pending snapshot")
     reader.free()
     world.free()
+    # Exercise the stage entry point too: manager tests alone do not catch a
+    # regression back to the synchronous API in stage.gd.
+    var shared_manager: Node = root.get_node("SaveManager")
+    var original_directory: String = shared_manager.save_directory
+    shared_manager.save_directory = directory
+    var stage = load("res://stage/stage.gd").new()
+    stage.pending_page = 2
+    stage.owniverse.state.energy = 42.0
+    stage.save_data()
+    check(shared_manager.is_saving(), "Stage queues a background save instead of waiting for disk")
+    stage.owniverse.state.energy = 43.0
+    shared_manager.flush_saves()
+    check(shared_manager.load_data("owniverse", "qwerty").state.energy == 42.0, "Stage save writes an independent snapshot")
+    stage.owniverse.free()
+    stage.free()
+    shared_manager.save_directory = original_directory
     global.save_plaintext_copy = old_plaintext
     for filename in DirAccess.get_files_at(directory):
         DirAccess.remove_absolute(directory.path_join(filename))