Typed owed vessels, ship lab building type, shield vessel gate

Groundwork for #5. vessels_owed carries the VESSEL TYPE rather than a
bare count, so a shield vessel finishing while the player is on a planet
comes back as a shield vessel. Fleet.add_ship takes a type, and vessels
already flying are re-statted when an upgrade is researched.

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

GameState.gd

@@ -104,10 +104,13 @@ const BUILDING_EXTRACTOR: String = "extractor"
 # Hosts the transport vessels that supply other buildings (#18). Built in
 # space like the rest; it is the transporters (#19) that do the moving.
 const BUILDING_TRANSPORT_HUB: String = "transport_hub"
+# Where vessel types are researched and new ones unlocked (#5).
+const BUILDING_SHIP_LAB: String = "ship_lab"
 
 const COMMAND_CENTER_COST: int = 15
 const EXTRACTOR_COST: int = 25
 const TRANSPORT_HUB_COST: int = 30
+const SHIP_LAB_COST: int = 35
 
 var buildings: Array = []
 
@@ -145,6 +148,11 @@ const SKILL_TREE: Array = [
 		"x": 0.18, "unlocks": BUILDING_TRANSPORT_HUB,
 		"desc": "Build transport hubs, which supply your other buildings with ore.",
 	},
+	{
+		"id": "vessel_science", "name": "Vessel Science", "cost": 35, "requires": "shipyards",
+		"x": 0.62, "unlocks": BUILDING_SHIP_LAB,
+		"desc": "Build ship labs, where each vessel type is upgraded and new ones unlocked.",
+	},
 	{
 		"id": "freight_yards", "name": "Freight Yards", "cost": 80, "requires": "freight",
 		"x": 0.05, "hub_bonus": 1,
@@ -269,12 +277,12 @@ func reset_progression() -> void:
 # on the same spot.
 var next_building_id: int = 1
 
-# Ships that have been paid for and built but not yet handed to the fleet.
-# Production is simulated here, in the autoload, the same way extractor
-# haulers are - so an order can complete while the player is down on a
-# planet's surface, where there is no Fleet node to hand a ship to. The fleet
-# claims them when it next can.
-var ships_owed: int = 0
+# Vessels that have been paid for and built but not yet handed to the fleet,
+# as a list of VESSEL_* types - a shield vessel finishing while the player is
+# on a planet must not come back as an interceptor. Production is simulated
+# here, in the autoload, the same way extractor haulers are, so an order can
+# complete where there is no Fleet node to hand a ship to.
+var vessels_owed: Array = []
 
 signal order_completed(building_id: int, type: String)
 
@@ -372,6 +380,33 @@ var vessel_levels: Dictionary = {}
 signal vessel_upgraded(upgrade_id: String)
 
 
+func has_ship_lab() -> bool:
+	for b in buildings:
+		if b.type == BUILDING_SHIP_LAB:
+			return true
+	return false
+
+
+func shield_vessel_unlocked() -> bool:
+	# Unlocked AT the lab, by paying for it there - a vessel type rather than
+	# a stat, so it is a one-off rather than a level.
+	return int(vessel_levels.get(UNLOCK_SHIELD_VESSEL, 0)) > 0
+
+
+const UNLOCK_SHIELD_VESSEL: String = "unlock_shield"
+const UNLOCK_SHIELD_COST: int = 45
+
+
+func unlock_shield_vessel() -> bool:
+	if shield_vessel_unlocked() or not has_ship_lab():
+		return false
+	if not spend(UNLOCK_SHIELD_COST):
+		return false
+	vessel_levels[UNLOCK_SHIELD_VESSEL] = 1
+	vessel_upgraded.emit(UNLOCK_SHIELD_VESSEL)
+	return true
+
+
 func upgrade_def(id: String) -> Dictionary:
 	for u in VESSEL_UPGRADES:
 		if u.id == id:
@@ -404,7 +439,7 @@ func upgrade_cost(id: String) -> int:
 
 
 func can_buy_upgrade(id: String) -> bool:
-	return not upgrade_maxed(id) and can_afford(upgrade_cost(id))
+	return has_ship_lab() and not upgrade_maxed(id) and can_afford(upgrade_cost(id))
 
 
 func buy_upgrade(id: String) -> bool:
@@ -531,6 +566,8 @@ func queue_order(id: int, product: String = PRODUCT_SHIP) -> bool:
 	var b: Dictionary = find_building(id)
 	if b.is_empty() or not can_produce(b.type):
 		return false
+	if product == VESSEL_SHIELD and not shield_vessel_unlocked():
+		return false
 	if product == PRODUCT_TRANSPORTER and not has_transport_hub():
 		# Nowhere to station it. Refused rather than queued, so the player is
 		# told now instead of watching an order sit forever.
@@ -570,7 +607,7 @@ func _advance_production() -> void:
 			if product == PRODUCT_TRANSPORTER:
 				_station_transporter(b.position)
 			else:
-				ships_owed += 1
+				vessels_owed.append(product)
 			order_completed.emit(int(b.id), product)