feat(multiplayer): notify clients before server shutdown

Game/scripts/match_net.gd

@@ -17,6 +17,7 @@ signal player_left(peer_id: int)
 signal player_state_changed(peer_id: int, team: int, ready: bool)
 signal rejected(reason: String)   # client-side only: the server refused our hello
 signal welcomed()                 # client-side only: our hello was accepted
+signal server_shutdown(reason: String) # client-side notification before planned close
 
 const TEAM_COUNT := 2
 const RECONNECT_GRACE_SECONDS := 60.0
@@ -215,6 +216,26 @@ func _broadcast_player_left(peer_id: int) -> void:
 			_player_left.rpc_id(other_peer_id, peer_id)
 
 
+func broadcast_server_shutdown(reason: String) -> void:
+	if not multiplayer.is_server():
+		return
+	var safe_reason := _sanitize_shutdown_reason(reason)
+	for peer_id in multiplayer.get_peers():
+		_server_shutdown.rpc_id(peer_id, safe_reason)
+
+
+static func _sanitize_shutdown_reason(raw: String) -> String:
+	var clean := ""
+	for c in raw:
+		var code := c.unicode_at(0)
+		if code >= 0x20 and code != 0x7F:
+			clean += c
+	clean = clean.strip_edges()
+	if clean.length() > 96:
+		clean = clean.substr(0, 96)
+	return clean if not clean.is_empty() else "server_shutdown"
+
+
 # Balances a new joiner onto whichever team currently has fewer players
 # (ties go to team 0). Server only.
 func _pick_balanced_team() -> int:
@@ -488,6 +509,11 @@ func _rejected(reason: String) -> void:
 	rejected.emit(reason)
 
 
+@rpc("authority", "call_remote", "reliable")
+func _server_shutdown(reason: String) -> void:
+	server_shutdown.emit(_sanitize_shutdown_reason(reason))
+
+
 @rpc("authority", "call_remote", "reliable")
 func _player_joined(peer_id: int, player_name: String, team: int, ready: bool) -> void:
 	roster[peer_id] = PlayerInfo.new(peer_id, player_name, team, ready)