Supply a building by hand: withdraw at the mothership, unload on arrival

Forgejo #17, part 4 of #12. #16 lets an order queue waiting for ore, so
something has to be able to fulfil one. Until transporters exist (#19)
this is the only route, which is why it lands before them rather than
after: otherwise a player with a queued order and no Transport Hub is
stuck forever.

A "Load ore" button at the mothership moves a batch from the bank into
the fleet's hold, and opening a building hands it whatever the fleet is
carrying.

Delivery at the mothership is now EDGE triggered on arrival rather than
running every frame the fleet is parked there. Continuous delivery made
withdrawing impossible - ore loaded into the hold was banked again on the
very next frame, so the button would have looked broken. A batch rather
than the whole bank, because the hold is the part that can be lost and
emptying the bank in one tap would also strand research, which still
spends from it.

Both new logistics tests now clear existing asteroids: disabling
AsteroidField only stops new chunks spawning, and a drifting rock
destroying a parked ship changed the fleet size the tests assert on.
That was a real intermittent failure, seen once in a full run.

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

GameState.gd

@@ -45,6 +45,35 @@ func collect_cargo(amount: int = 1) -> void:
 	fleet_cargo += amount
 
 
+# One press of "load ore" at the mothership. A batch rather than the whole
+# bank: the hold is the part that can be lost, and emptying the bank in one
+# tap would also strand research, which still spends from it.
+const CARGO_LOAD_BATCH: int = 25
+
+
+func withdraw_to_fleet(amount: int = CARGO_LOAD_BATCH) -> int:
+	# Banked ore back into the hold, so the fleet can carry it to a building
+	# that is waiting on it (#17).
+	var taken: int = mini(amount, rocks)
+	if taken <= 0:
+		return 0
+	rocks -= taken
+	fleet_cargo += taken
+	return taken
+
+
+func unload_into_building(id: int) -> int:
+	# Everything the fleet is carrying, into this building's stock. Returns
+	# how much moved so a caller can tell the player something happened.
+	if fleet_cargo <= 0:
+		return 0
+	var moved: int = fleet_cargo
+	if deposit_to_building(id, moved) <= 0:
+		return 0
+	fleet_cargo = 0
+	return moved
+
+
 func deliver_cargo() -> int:
 	# Everything the fleet is carrying, banked at once. Returns how much moved
 	# so a caller can react to a delivery actually having happened rather than