Cap fleet cargo capacity by composition (#56)

The fleet hold was unbounded - GameState.fleet_inventory had no
total_capacity, so a lone ship could carry unlimited resources. Capacity
is now data-driven: only vessels declaring the "cargo" capability
contribute their cargo_capacity stat (interceptor: 20; shield vessel and
gas bomber: 0, unarmed transporters are never in the fleet at all).
Fleet.gd recomputes it in the one place composition can change
(spawn_ships/add_ship/store_ship/_on_ship_destroyed), which also covers
the space<->surface scene-swap restore since that goes through
spawn_ships().

Rock.gd and GasVentField.gd (the two "fly over it to collect" pickup
paths) now check what collect_cargo()/collect_gas_cargo() actually
accepted before consuming their source, instead of assuming an
unlimited hold. The HUD's FLEET HOLD caption shows the live total/cap.

Also fixes test_resource_transfer.gd, which pushed more ore through a
2-ship fleet than the new cap allows - it now spawns the largest fleet
every unlocked doctrine permits, since that test is about the transfer
UI rather than the cap itself (test_fleet_cargo_limit.gd owns that).

tests/_fleet_cargo_limit_preview.gd

@@ -0,0 +1,19 @@
+extends Node2D
+
+# Preview, not a test - the fleet-hold cap shown on the FLEET HOLD caption.
+#
+#   bash tests/screenshot.sh res://tests/_fleet_cargo_limit_preview.tscn /tmp/cargo_limit.png 15 720x1280
+
+
+func _ready() -> void:
+	GameState.buildings.clear()
+	GameState.extractors.clear()
+	GameState.rocks = 200
+	GameState.gas = 30
+	GameState.fleet_cargo = 0
+	var scene: Node = load("res://Main.tscn").instantiate()
+	add_child(scene)
+	var fleet: Node = scene.get_node("Fleet")
+	fleet.spawn_ships(3, Vector2.ZERO, PackedStringArray(["interceptor", "interceptor", "shield"]))
+	# Two cargo-capable interceptors (40) + one shield vessel (0) = 40 total.
+	GameState.fleet_cargo = 27