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.

CarrierCapacityUpgrade.tres

@@ -0,0 +1,11 @@
+[gd_resource type="Resource" script_class="VesselUpgradeDefinition" load_steps=2 format=3]
+[ext_resource type="Script" path="res://VesselUpgradeDefinition.gd" id="1"]
+[resource]
+script = ExtResource("1")
+upgrade_id = "car_capacity"
+display_name = "Reinforced Hold"
+description = "+30 cargo capacity per level."
+base_cost = 40
+max_level = 3
+icon = 21
+additive_modifiers = {"cargo_capacity": 30.0}