Cap fleet cargo capacity by composition (#56)

The fleet hold was unbounded - GameState.fleet_inventory had no
total_capacity, so a lone ship could carry unlimited resources. Capacity
is now data-driven: only vessels declaring the "cargo" capability
contribute their cargo_capacity stat (interceptor: 20; shield vessel and
gas bomber: 0, unarmed transporters are never in the fleet at all).
Fleet.gd recomputes it in the one place composition can change
(spawn_ships/add_ship/store_ship/_on_ship_destroyed), which also covers
the space<->surface scene-swap restore since that goes through
spawn_ships().

Rock.gd and GasVentField.gd (the two "fly over it to collect" pickup
paths) now check what collect_cargo()/collect_gas_cargo() actually
accepted before consuming their source, instead of assuming an
unlimited hold. The HUD's FLEET HOLD caption shows the live total/cap.

Also fixes test_resource_transfer.gd, which pushed more ore through a
2-ship fleet than the new cap allows - it now spawns the largest fleet
every unlocked doctrine permits, since that test is about the transfer
UI rather than the cap itself (test_fleet_cargo_limit.gd owns that).

tests/test_resource_transfer.gd

@@ -88,7 +88,12 @@ func _process(delta: float) -> bool:
 			scene.get_node("AsteroidField").set_process(false)
 			scene.get_node("EnemySpawner").set_process(false)
 			_clear_hazards()
-			fleet.spawn_ships(2, MOTHERSHIP_AT)
+			# #56 gave the hold a finite, composition-driven capacity. This
+			# test is about the transfer UI, not that cap - test_fleet_cargo_
+			# limit.gd owns the cap itself - so it spawns the largest fleet
+			# every unlocked doctrine allows (8 interceptors x 20 = 160),
+			# comfortably above every hand-picked amount below.
+			fleet.spawn_ships(game_state.max_fleet_size(), MOTHERSHIP_AT)
 			var field: Node = scene.get_node("BuildingField")
 			field.place(GameState.BUILDING_MOTHERSHIP, MOTHERSHIP_AT)
 			mothership_id = int(game_state.buildings[-1].id)