Add carrier vessel (#57)

An unarmed hauler with no weapon component - only VesselCargoComponent,
so it structurally cannot fire, the same pattern that already made the
shield vessel weaponless after #35. Its cargo_capacity (120, six
interceptors' worth) makes it the first real payoff for #56's fleet
hold cap.

Wired into the crafter, hangar and ship lab menus, all of which were
already generic over VesselDefinitions.fleet()/.all() - only the
per-vessel UI *nodes* are hand-placed (Main.tscn), not the logic, so
this repacks those three button rows to 4/4/5 columns and extends
BuildController's matching onready arrays. No new signal handlers were
needed.

Also fixes test_vessel_composition.gd's hardcoded fleet-catalog size
(3 -> 4) now that a fourth vessel is fleet-eligible.

GameState.gd

@@ -463,6 +463,7 @@ const VESSEL_INTERCEPTOR: String = "interceptor"
 const VESSEL_SHIELD: String = "shield"
 const VESSEL_GAS_BOMBER: String = "gas_bomber"
 const VESSEL_TRANSPORTER: String = "transporter"
+const VESSEL_CARRIER: String = "carrier"
 
 # Compatibility constants for callers and old tests. Runtime metadata is
 # resolved from VesselDefinition below.