Clear designation tile-highlight when jobs complete

Each entity completion handler (wall/floor/door/bed/torch/workbench/crate
/tree/rock/big_rock/grave_slot) now calls World.clear_designation_at(tile)
so the orange/blue/etc. highlight overlay disappears with the job.
BigRock iterates its footprint to clear all four tiles.

World.designation_ctl is set during the scene boot wire-up; the helper
no-ops when the controller is absent (e.g. headless tests).

scenes/entities/door.gd

@@ -181,4 +181,5 @@ func _complete() -> void:
 	if sprite != null:
 		sprite.modulate.a = 1.0
 	queue_redraw()
+	World.clear_designation_at(tile)
 	Audit.log("door", "door completed at %s" % tile)