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.

CarrierVesselDefinition.tres

@@ -0,0 +1,25 @@
+[gd_resource type="Resource" script_class="VesselDefinition" load_steps=4 format=3]
+[ext_resource type="Script" path="res://VesselDefinition.gd" id="1"]
+[ext_resource type="PackedScene" path="res://VesselCargoComponent.tscn" id="2"]
+[ext_resource type="Resource" path="res://CarrierCapacityUpgrade.tres" id="3"]
+[resource]
+script = ExtResource("1")
+definition_id = "carrier"
+display_name = "Carrier"
+description = "Unarmed hauler with a much larger hold than any combat vessel. Cannot fire."
+build_cost = 24
+build_time = 20.0
+icon = 34
+glow_color = Color(1, 0.78, 0.3, 1)
+hull_points = PackedVector2Array(-32, -10, -18, -18, 18, -18, 32, -10, 32, 14, -32, 14)
+fleet_eligible = true
+capabilities = PackedStringArray("cargo")
+component_scenes = Array[PackedScene]([ExtResource("2")])
+base_stats = {"max_speed": 380.0, "acceleration": 360.0, "delivery_speed": 180.0, "cargo_capacity": 120.0}
+upgrades = Array[Resource]([ExtResource("3")])
+unlock_id = "unlock_carrier"
+unlock_name = "Unlock Carrier"
+unlock_description = "Research a dedicated hauler hull that trades weapons for a much larger cargo hold."
+unlock_cost = 0
+unlock_resource_costs = {"ore": 60}
+unlock_icon = -1