Add gas giant resource system

tests/test_gas_system.gd

@@ -0,0 +1,64 @@
+extends SceneTree
+
+# Forgejo #27: gas giants are deterministic animated gas sources, and the
+# existing transporter route carries their gas without converting it to ore.
+
+var game_state: Node
+var stage: int = 0
+var elapsed: float = 0.0
+
+
+func _initialize() -> void:
+	game_state = root.get_node("GameState")
+	game_state.buildings.clear()
+	game_state.extractors.clear()
+	game_state.transporters.clear()
+	game_state.gas = 0
+	game_state.rocks = 0
+
+
+func _process(delta: float) -> bool:
+	elapsed += delta
+	if stage == 0:
+		var found_giant: bool = false
+		for x in range(-12, 13):
+			for y in range(-12, 13):
+				var data: Dictionary = WorldGen.planet_data_at_chunk(Vector2i(x, y))
+				if String(data.get("planet_type", "")) == "gas_giant":
+					found_giant = String(data.get("resource_type", "")) == "gas"
+					break
+			if found_giant:
+				break
+		if not found_giant:
+			return _fail("world generation produced no deterministic gas giant")
+		var giant: Node2D = load("res://Planet.tscn").instantiate()
+		giant.planet_type = "gas_giant"
+		giant._process(0.5)
+		if giant.gas_phase <= 0.0:
+			return _fail("gas giant animation did not advance")
+		giant.free()
+
+		game_state.add_building(GameState.BUILDING_MOTHERSHIP, Vector2.ZERO)
+		game_state.add_building(GameState.BUILDING_TRANSPORT_HUB, Vector2(100, 0))
+		game_state.add_extractor("Aurelia", Vector2(400, 0), "gas")
+		game_state.extractors[0].stock = game_state.transporter_load_limit()
+		game_state.transporters.append({"hub_id": 2, "state": GameState.TRANSPORTER_TO_EXTRACTOR,
+			"target_id": 0, "extractor_index": 0, "carrying": 0, "resource_type": "gas",
+			"progress": 1.0, "from": Vector2(100, 0), "to": Vector2(400, 0)})
+		game_state._arrive(game_state.transporters[0])
+		if game_state.transporters[0].state != GameState.TRANSPORTER_TO_MOTHERSHIP_FROM_EXTRACTOR:
+			return _fail("transporter did not collect gas from the extractor")
+		game_state._arrive(game_state.transporters[0])
+		if game_state.gas != game_state.transporter_load_limit() or game_state.rocks != 0:
+			return _fail("gas delivery banked gas=%d ore=%d" % [game_state.gas, game_state.rocks])
+		print("gas giant animated; transporter banked %d gas without creating ore" % game_state.gas)
+		print("TEST_GAS_SYSTEM: PASS")
+		quit(0)
+		return true
+	return false
+
+
+func _fail(message: String) -> bool:
+	print("TEST_GAS_SYSTEM: FAIL (%s)" % message)
+	quit(1)
+	return true