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/workbench.gd

@@ -419,6 +419,7 @@ func _complete() -> void:
 	if _light != null:
 		_light.enabled = is_on()
 	queue_redraw()
+	World.clear_designation_at(tile)
 	Audit.log("workbench", "%s built at %s" % [label_text, tile])
 	# Phase 13 — notify BeautySystem so nearby tile beauty scores update.
 	# Hearth gets base beauty 4 (warm glow); other benches get 1.