Remove the one-way ore API superseded by the transfer menu

The loose end from #24. withdraw_to_fleet, unload_into_building and
deliver_cargo were the old one-way moves behind "Load ore" and "Store
ore here"; every one of them is a special case of transfer_to_building /
transfer_to_fleet, and none had a caller left outside tests. The
cargo_delivered signal had no listeners at all.

Not just tidying: unload_into_building routed through
deposit_to_building, which does NOT special-case the mothership - it
writes b.stock, while building_holdings() returns `rocks` for a
mothership and ignores stock entirely. So unload_into_building(
mothership_id) would have parked ore in a field nothing reads, with no
error. Nothing called it that way, but it is a plausibly-named function
sitting there to be reached for later. transfer_to_building handles it.

CARGO_LOAD_BATCH stays - the transfer menu's batch buttons use it; only
its role as withdraw_to_fleet's default argument is gone.

The #24 note expected folding these in to be costly for
test_cargo_readout, which places no buildings. It is not: the transfer
primitives work off GameState records rather than spawned nodes, so a
single add_building() in setup is enough and it stays a display test.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/_transfer_menu_preview.gd

@@ -0,0 +1,27 @@
+extends Node2D
+
+# Preview, not a test. Opens the TransferMenu (#24) so its layout can be
+# checked on a real viewport - four buttons, two captions and the shared
+# header have to coexist without collisions, which no headless test can see.
+#
+#   bash tests/screenshot.sh res://tests/_transfer_menu_preview.tscn /tmp/m.png 20 720x1280
+#   ... and again at 720x960 to check a shorter screen format.
+
+
+func _ready() -> void:
+	GameState.buildings.clear()
+	GameState.extractors.clear()
+	GameState.transporters.clear()
+	GameState.rocks = 180
+	GameState.fleet_cargo = 24
+	GameState.unlocked = ["shipyards", "logistics", "freight"]
+	var main: Node = load("res://Main.tscn").instantiate()
+	add_child(main)
+	main.get_node("BuildingField").place(GameState.BUILDING_SHIP_CRAFTER, Vector2(400.0, 0.0))
+	var crafter_id: int = int(GameState.buildings[-1].id)
+	GameState.deposit_to_building(crafter_id, 42)
+	var bc: Node = main.get_node("BuildController")
+	bc.transfer_building_id = crafter_id
+	main.get_node("UI/TransferMenu").visible = true
+	bc._refresh_transfer()
+	bc.transfer_status.text = "Moved 25 ore into this building"