Save The Tree implementation and acceptance work in progress

Preserve production code, regression fixtures, audio assets and provenance, test infrastructure, and the original-completion plan and progress ledger. Final game and Windows acceptance remain pending; excluded local artifacts and native experiments are not part of this snapshot.

tests/transport/test_runtime_transport.gd

@@ -1,5 +1,8 @@
 extends SceneTree
 
+const FixtureCleanup := preload("res://tests/helpers/ui_audio_fixture_cleanup.gd")
+var _finishing := false
+
 var _checks := 0
 var _failures := 0
 var _runtime: Variant
@@ -187,5 +190,13 @@ func _expect(condition: bool, message: String) -> void:
 	print("%s %s" % ["PASS" if condition else "FAIL", message])
 
 func _finish() -> void:
+	if _finishing:
+		return
+	_finishing = true
+	if is_instance_valid(_main):
+		var final_cleanup: Dictionary = await FixtureCleanup.free_owner_and_wait(self, _main)
+		if not bool(final_cleanup.get(&"passed", false)):
+			_failures += 1
+			push_error("ordinary Main final audio cleanup failed")
 	print("Runtime transport: %d checks, %d failures" % [_checks, _failures])
 	quit(1 if _failures else 0)