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).

Rock.gd

@@ -82,7 +82,10 @@ func _on_area_entered(area: Area2D) -> void:
 	if area.is_in_group("player_ships"):
 		# Into the fleet's hold, not the bank (#14). It only counts as
 		# spendable once the fleet has flown it back to the mothership.
-		GameState.collect_cargo(1)
+		# #56: the hold has a finite total_capacity now - a full fleet must
+		# leave the rock behind rather than despawning it for nothing.
+		if GameState.collect_cargo(1) <= 0:
+			return
 		collected.emit()
 		queue_free()