Transporter capacity per hub, and research to raise it

Forgejo #20, part 7 of #12 and the last of it. A hub stations
BASE_HUB_CAPACITY (2) transporters. Total berths are hubs x per-hub
capacity, so building another hub or researching "Freight Yards" both
raise it - which is the choice #12 asks the player to make.

Capacity follows the same shape as max_fleet_size() (sum a bonus over
`unlocked`) rather than being a second mechanism doing the same job, so
research applies to hubs already built, not only to ones placed
afterwards.

A queued-but-unbuilt transporter holds its berth. Without that a player
could order ten against two berths and only discover the problem when
they refused to appear. Ordering past the cap is refused outright, and
the two refusals - no hub at all, versus every berth taken - say which,
because "nothing happened" is exactly the bug reported against the
craft button.

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

GameState.gd

@@ -145,6 +145,11 @@ const SKILL_TREE: Array = [
 		"x": 0.18, "unlocks": BUILDING_TRANSPORT_HUB,
 		"desc": "Build transport hubs, which supply your other buildings with ore.",
 	},
+	{
+		"id": "freight_yards", "name": "Freight Yards", "cost": 80, "requires": "freight",
+		"x": 0.05, "hub_bonus": 1,
+		"desc": "Permanent: +1 transporter stationed at every hub, including ones already built.",
+	},
 	{
 		"id": "wing", "name": "Wing Doctrine", "cost": 30, "requires": "coordination",
 		"x": 0.82, "fleet_bonus": 1,
@@ -357,6 +362,9 @@ func queue_order(id: int, product: String = PRODUCT_SHIP) -> bool:
 		# Nowhere to station it. Refused rather than queued, so the player is
 		# told now instead of watching an order sit forever.
 		return false
+	if product == PRODUCT_TRANSPORTER and not can_station_transporter():
+		# Every berth is taken or spoken for (#20).
+		return false
 	(b.orders as Array).append(product)
 	return true
 
@@ -374,6 +382,11 @@ func _advance_production() -> void:
 			var cost: int = product_cost(product)
 			if int(b.get("stock", 0)) < cost:
 				break
+			if product == PRODUCT_TRANSPORTER and transporters.size() >= transport_capacity():
+				# A hub was demolished after this was ordered, so the berth it
+				# was going to use is gone. Hold the order rather than
+				# overfilling what is left.
+				break
 			if product == PRODUCT_TRANSPORTER and nearest_hub_to(b.position).is_empty():
 				# Every hub was demolished while this sat in the queue. Leave
 				# it queued and the stock untouched rather than producing a
@@ -548,6 +561,11 @@ func _process(delta: float) -> void:
 # "from", "to"}.
 const TRANSPORTER_SPEED: float = 340.0
 
+# How many transporters one hub can station. Raised for EVERY hub - built
+# already or not - by research, which is the same shape as max_fleet_size()
+# rather than a second mechanism doing the same job.
+const BASE_HUB_CAPACITY: int = 2
+
 const TRANSPORTER_IDLE: String = "idle"
 const TRANSPORTER_TO_MOTHERSHIP: String = "to_mothership"
 const TRANSPORTER_TO_BUILDING: String = "to_building"
@@ -556,6 +574,48 @@ const TRANSPORTER_RETURNING: String = "returning"
 var transporters: Array = []
 
 
+func hub_capacity() -> int:
+	# Per hub.
+	var n: int = BASE_HUB_CAPACITY
+	for id in unlocked:
+		n += int(skill_node(id).get("hub_bonus", 0))
+	return n
+
+
+func hub_count() -> int:
+	var n: int = 0
+	for b in buildings:
+		if b.type == BUILDING_TRANSPORT_HUB:
+			n += 1
+	return n
+
+
+func transport_capacity() -> int:
+	# Total berths: more hubs, or more capacity per hub. Either raises it,
+	# which is exactly the choice #12 asks the player to make.
+	return hub_count() * hub_capacity()
+
+
+func pending_transporter_orders() -> int:
+	var n: int = 0
+	for b in buildings:
+		for o in b.get("orders", []):
+			if o == PRODUCT_TRANSPORTER:
+				n += 1
+	return n
+
+
+func transporters_claimed() -> int:
+	# Built plus already ordered: an order in the queue has a berth reserved,
+	# or a player could queue ten of them against two berths and only find out
+	# when they refused to appear.
+	return transporters.size() + pending_transporter_orders()
+
+
+func can_station_transporter() -> bool:
+	return transporters_claimed() < transport_capacity()
+
+
 func has_transport_hub() -> bool:
 	return not nearest_hub_to(Vector2.ZERO).is_empty()