Fix simulation_server_stub Entity class conflict + ServerConfig load timing

- Renamed inner class Entity→StubEntity (Godot has native Entity class)
- Added await ServerConfig.config_loaded before reading map_list in server_main.gd
- Config loads via call_deferred, so _ready() must wait for signal

scripts/network/server_main.gd

@@ -40,6 +40,10 @@ var _game_server: Node = null
 # Lifecycle
 # ---------------------------------------------------------------------------
 func _ready() -> void:
+	# Wait for ServerConfig to finish (it loads via call_deferred)
+	if ServerConfig and ServerConfig.has_signal(&"config_loaded"):
+		await ServerConfig.config_loaded
+
 	# Config driven — ServerConfig singleton loaded at autoload time.
 	# Allow port override via env var for backwards compatibility
 	# (container/VPS deployments that set SERVER_PORT).