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/world/crate.gd

@@ -143,6 +143,7 @@ func on_build_tick() -> void:
 	if build_progress >= BUILD_TICKS:
 		_completed = true
 		emit_signal("contents_changed")
+		World.clear_designation_at(tile)
 		Audit.log("crate", "built at %s (capacity %d)" % [tile, CAPACITY])
 	queue_redraw()