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_stair_transport_details.gd

@@ -1,4 +1,5 @@
 extends SceneTree
+const FixtureCleanup := preload("res://tests/helpers/ui_audio_fixture_cleanup.gd")
 
 const LegacyType := preload("res://scripts/world/legacy_stair_service.gd")
 
@@ -155,6 +156,11 @@ func _click(at: Vector2) -> void:
 
 func _finish() -> void:
 	_clock.set_paused(true)
+	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("Stair and transport details UI: %d checks, %d failures" % [_checks,_failures])
 	quit(_failures)