feat: Quick match correctness and realtime protocol

[tintinhamans]

Quick match could start short of players or with players who weren't connected, clients had no reliable join order, and only the host saw why a start failed.

• The match is re-verified before the countdown and before start: same players, all connections live, no membership change since the check.
• Playlists have a min/max range, and maps always have exactly as many slots as players. Players are told when a fallback map or a smaller match is used. Bad playlists are rejected at startup.
• Custom lobby and quick match connectivity checks are refereed the same way: pass once everyone is connected, fail as soon as a client reports gave_up, 20 s backstop (full_mesh_check_referee_window_ms). A failed custom check re-signals the missing pairs so the next start attempt can pass.
• Everyone in the lobby sees the connection check and who can't connect to whom.
• Lobby fixes: a host can't kick themselves, DELETE only leaves the named lobby, creating a lobby no longer deletes one other players are in, a full lobby is rejected before leaving the current one, slots a failed start check closed are reopened, host migration never shows a member twice, stale legacy mesh-check replies are ignored, and a missing has_map no longer fails the join.
• The search widens the skill range every 10 s (shown as ±N), and searches all skill ranges past ±1500.
• Additive fields: JoinSequence, the lobby snapshot in "match found", the mesh-check reason, game CRCs on the session (409 on custom-join mismatch), per-player ice_implementation with a tester allowlist.
• Compatible with the released client.