Issue: Disconnection Detection Failure with Stealth Clients

When only stealth clients remain in a room, the server fails to send an "empty room broadcast." Consequently, disconnection detection is never triggered on the client side.

Reproduction Flow

1. Room State: Stealth Client A (Local) + Normal Client B (Remote).
2. Client B Disconnects: After 5 seconds, the server’s _cleanup_clients removes B.
3. Room Flags as "Dirty": _adaptive_broadcast_all_rooms attempts a broadcast.
4. Snapshot Construction: During the client_snapshot build, Stealth Client A is skipped, resulting in an empty list.
5. Serialization Logic: _serialize_room_transform is called, but execution hits the following guard clause:

# server.py:1810-1811
if not client_snapshot:
    return None  # Ends here without sending the broadcast

6. Unity Side: The Unity client never receives a room broadcast that excludes Client B.
7. Logic Failure: The disconnection detection logic in ProcessRoomTransform (MessageProcessor.cs:339-365) is never triggered.
8. Ghosting: Client B’s remote avatar remains stuck in the Hierarchy.

Why it works in "Normal Mode"

In Normal Mode (where LocalAvatarPrefab is present), the local client is treated as a non-stealth client.

After Client B disconnects, the local client's transform data is still included in the client_snapshot. Because the snapshot is not empty, the broadcast is successfully sent, allowing the remaining client to recognize that B is no longer in the room list and clean up the avatar.