Fleet carries mined ore until it delivers it to the mothership

Forgejo #14, part 1 of #12. Rock.gd credited GameState.rocks on pickup,
so ore was banked instantly from anywhere on the map and a mining run
had nothing at stake. Ore now goes into GameState.fleet_cargo and only
becomes spendable when the fleet reaches the mothership.

Delivery lives in BuildingField because that node already knows where
the buildings are and already answers "is the fleet standing on one?",
using the fleet CENTROID - the same stable single-number test Planet
and Building use, rather than per-ship collision that flickers as ships
jostle in formation.

Cargo lives in the GameState autoload so it survives Main.tscn being
torn down on every trip to a planet's surface - cargo mined ON a
surface would otherwise be dropped the moment the player took off.

test_resources_and_buildings asserted the old behaviour (gathering a
rock raises `rocks`) and now asserts the new one. That is a spec
change, not a weakened test - the gathering assertion itself is intact.

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

Rock.gd

@@ -80,7 +80,9 @@ func _nearest_ship() -> Node2D:
 
 func _on_area_entered(area: Area2D) -> void:
 	if area.is_in_group("player_ships"):
-		GameState.rocks += 1
+		# Into the fleet's hold, not the bank (#14). It only counts as
+		# spendable once the fleet has flown it back to the mothership.
+		GameState.collect_cargo(1)
 		collected.emit()
 		queue_free()