Buildings produce from their own stock, and queue orders until supplied

Forgejo #16, part 3 of #12. BuildController deducted the cost from the
bank at the moment of purchase and refused outright with "Not enough
rocks", so nothing ever had to be delivered anywhere. Each building now
carries its own stock, and an order it cannot pay for yet is QUEUED and
completes by itself once ore arrives.

Records get an `id` because a building has to be referable across the
space <-> planet-surface scene swap, when every Building NODE has been
destroyed and rebuilt. Position would nearly work and then fail on two
buildings placed on the same spot.

Production advances in the GameState autoload, the same way extractor
haulers already do, so an order can complete while the player is down a
gravity well. A finished vessel becomes `ships_owed` rather than being
handed straight to a Fleet that may not exist, and the fleet claims what
it is owed when it next has room.

The craft menu remembers which crafter it was opened for: _process()
clears current_building while any menu is open, so by the next frame the
menu would no longer know. It shows the ore delivered HERE rather than
the bank, since showing the bank would promise something the crafter
cannot actually build.

Ship crafting cannot be supplied until #17 lands - nothing yet moves ore
into a building.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Building.gd

@@ -19,6 +19,10 @@ extends Node2D
 # never changes were a real framerate cost earlier on.
 
 @export var building_type: String = "mothership"
+# Which GameState.buildings record this node is showing. Set by
+# BuildingField when it spawns, so a node can be traced back to the stock
+# and production queue that survive the scene swap (#16).
+@export var building_id: int = 0
 @export var interact_radius: float = 150.0
 
 const MOTHERSHIP_COLOR: Color = Color(0.45, 0.9, 0.55)