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_guest_observation.gd

@@ -1,4 +1,5 @@
 extends SceneTree
+const FixtureCleanup := preload("res://tests/helpers/ui_audio_fixture_cleanup.gd")
 var checks := 0
 var failures := 0
 func _initialize() -> void: call_deferred("run")
@@ -62,7 +63,11 @@ func run() -> void:
 		await process_frame
 		await RenderingServer.frame_post_draw
 		root.get_texture().get_image().save_png("res://artifacts/guest-ai-2026-09-29/observer.png")
-	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
 	await process_frame
 	print("Guest observer: %d checks, %d failures" % [checks, failures])
 	quit(failures)