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()
 

scenes/world/world.gd

@@ -185,6 +185,9 @@ func _ready() -> void:
 
 	# Designation: bind the paint surface + the Selection guard.
 	designation_ctl.bind(designation_layer, selection)
+	# Expose on the autoload so entities can clear their ghost stamp on
+	# completion (Tree.fell, Wall._complete, etc.).
+	World.designation_ctl = designation_ctl
 
 	# Register all 10 providers — Decision iterates by .priority desc.
 	# doctor=9 > sleep=8 > eat=7 > construction=6 > chop=5 ≈ plant=5 > mine=4