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/grave_slot.gd

@@ -224,4 +224,5 @@ func from_dict(d: Dictionary) -> void:
 func _complete_dig() -> void:
 	_dug = true
 	queue_redraw()
+	World.clear_designation_at(tile)
 	Audit.log("grave_slot", "grave dug at %s (ready for burial)" % tile)