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.

tests/_carrier_crafter_preview.gd

@@ -0,0 +1,17 @@
+extends Node2D
+
+# Preview, not a test - #57's crafter menu screenshot with all 4 fleet
+# vessels researched, carrier included as the new 4th entry.
+#
+#   bash tests/screenshot.sh res://tests/_carrier_crafter_preview.tscn /tmp/crafter.png 20 720x1280
+
+
+func _ready() -> void:
+	GameState.buildings.clear()
+	GameState.extractors.clear()
+	GameState.rocks = 500
+	GameState.vessel_levels = {"unlock_shield": 1, "unlock_gas_bomber": 1, "unlock_carrier": 1}
+	var main: Node = load("res://Main.tscn").instantiate()
+	add_child(main)
+	main.get_node("UI/CraftMenu").visible = true
+	main.get_node("BuildController")._refresh_craft_status()