fix player disconnect

[mercury233]

This change prevents the server from writing to a bufferevent that is already being disconnected, and clears stale duel-side player references when a player is removed.

It also marks single duel disconnect endings as DUEL_STAGE_END so remaining clients closing afterward do not trigger a second duel-end notification for observers.

[Copilot]

Pull request overview

Fixes use-after-free / stale-pointer issues that occurred when a networked player disconnects while a duel mode still holds raw DuelPlayer* pointers into the users map entry that's about to be erased. A new virtual hook lets each duel mode null out its references before the bufferevent is freed, and server-side send helpers now skip writes to the bufferevent currently being torn down.

Changes:

• Add pure virtual DuelMode::OnPlayerDisconnected() and implement it in SingleDuel and TagDuel to clear host_player, players[], pplayer[], ready[], and (tag) surrender[]/cur_player[], plus erase from observers.
• Track the currently-disconnecting bufferevent* in NetServer and gate all send helpers behind CanWriteToPlayer() to prevent writing into a buffer that's about to be freed (while still preserving last_sent so ReSendToPlayer works for remaining players).
• Invoke OnPlayerDisconnected() from DisconnectPlayer() before bufferevent_free(), and reset dp->game/state/type/bev so any subsequent accidental access is benign.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
gframe/network.h	Adds pure virtual OnPlayerDisconnected(DuelPlayer*) to DuelMode.
gframe/single_duel.h	Declares OnPlayerDisconnected override.
gframe/single_duel.cpp	Implements cleanup of host_player, players[], ready[], pplayer[], and observers.
gframe/tag_duel.h	Declares OnPlayerDisconnected override.
gframe/tag_duel.cpp	Implements cleanup of host_player, players[], ready[], surrender[], pplayer[], cur_player[], and observers.
gframe/netserver.h	Adds disconnecting_bev and CanWriteToPlayer(); routes all send helpers through it.
gframe/netserver.cpp	Defines disconnecting_bev; sets it around LeaveGame/DisconnectPlayer in the EOF/error path; calls OnPlayerDisconnected and resets DuelPlayer fields before freeing the bufferevent.

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[Copilot]

Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated no new comments.