Gas bombers consume gas from the fleet hold when firing (#60)

Shots cost GAS_PER_SHOT (5) from GameState.fleet_inventory, not the
bank - the fleet has to be carrying its own ammunition. The button's
enabled rule now also requires affordability, and a press on a ready-
but-unaffordable bomber still consumes the touch to explain itself via
a new status label (GasBombStatus, IconButton's own "greyed but still
pressable" convention) rather than either silently doing nothing or
consuming the bomber's cooldown for a shot that never fires.

Also documents a real, reproducible GDScript trap hit while writing
tests/test_gas_bomber_ammo.gd: a bare `ClassName.SOME_CONST` reference
anywhere in a -s entry test script can corrupt method resolution for
the WHOLE script, making an unrelated, earlier `some_node._process()`
call fail with "Nonexistent function '_process' in base 'Nil'" despite
the node being valid and the method existing (confirmed via
has_method()). Root cause unconfirmed; fixed by mirroring the constant
's value into a local const instead of referencing the class directly,
in all three gas-bomber tests.

tests/test_gas_bomber.gd and test_gas_bomber_cooldowns.gd updated to
stock fleet gas first, since shots are no longer free.

GameState.gd

@@ -104,6 +104,12 @@ func fleet_resource(resource_type: String) -> int:
 	return fleet_inventory.amount(resource_type)
 
 
+# #60: consumed outright (ammunition), not moved to another inventory like
+# transfer_resource_to_building/_fleet - returns the amount actually taken.
+func withdraw_fleet_resource(resource_type: String, amount: int) -> int:
+	return fleet_inventory.withdraw(resource_type, amount)
+
+
 # #56: total across every resource type in the hold, and the combined cap
 # that limits it - a HUD readout wants both, not any one resource's amount.
 func fleet_cargo_used() -> int: