feat(lobby): Tune the mesh check window and stop re-signalling connections that are still forming

[tintinhamans]

Members reply to a mesh check with a snapshot of their current connections, so an attempt used to be judged within milliseconds. Any connection still being negotiated was re-signalled, which throws away the negotiation, and the 5s window only mattered when a reply went missing. Attempts now re-poll snapshots every second until the mesh is complete or the window closes. Only then are missing pairs re-signalled, and pairs a member reports as still connecting (connecting_map) are left alone. Checks are judged on current members only, so a player who leaves mid-check no longer produces false gaps or stalls the attempt. Members who haven't replied are re-polled, which recovers replies lost to a reconnect.

The window, poll interval, retry delay and attempt count are tunable under Core (full_mesh_check_*, defaults 8000/1000/3000/2), and each judged attempt is logged with its duration and missing pairs so the window can be tuned from real data. Setup progress now carries countdown_ms so clients no longer infer the start countdown from timeout_ms. The mesh-check timeout stays at 10s or more, so older clients can't mistake it for the countdown. Older clients without connecting_map behave as before.

Pairs with the GameClient mesh tuning PR.