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/ui/test_upkeep_closed_ui.gd

@@ -1,4 +1,5 @@
 extends SceneTree
+const FixtureCleanup := preload("res://tests/helpers/ui_audio_fixture_cleanup.gd")
 ## Runs the real UI command handlers, a normal clock and disk save. Invoke again
 ## with -- --read and the same THE_TREE_UPKEEP_UI_SAVE for fresh-process restore.
 var checks := 0
@@ -130,6 +131,10 @@ func _capture() -> void:
 	var suffix := "read" if "--read" in OS.get_cmdline_user_args() else "write"
 	expect(root.get_texture().get_image().save_png("res://artifacts/upkeep-2026-09-29/closed-ui-%s.png" % suffix) == OK, "native rendered evidence saved")
 func _finish() -> void:
-	main.queue_free()
+	var _audio_cleanup_final: Dictionary = await FixtureCleanup.free_owner_and_wait(self, main)
+	if not bool(_audio_cleanup_final.get(&"passed", false)):
+		push_error("Ordinary fixture audio cleanup failed: %s" % str(_audio_cleanup_final))
+		quit(96)
+		return
 	print("%s: %d checks (phase %s)" % ["PASS" if failures == 0 else "FAIL", checks, "read" if "--read" in OS.get_cmdline_user_args() else "write"])
 	quit(failures)