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/tree.gd

@@ -119,6 +119,7 @@ func fell() -> void:
 	Audit.log("tree", "felled at %s; %d wood drops" % [tile, drops_count])
 	if Audio != null:
 		Audio.play_sfx(&"tree_fell")
+	World.clear_designation_at(tile)
 	queue_free()