onWGDisconnected only checked conn.ctx (the engine-scoped context), never
the watcher's own context. disableWgWatcherIfNeeded cancels the wgWatcherCtx,
not conn.ctx, so a disabled watcher's timeout callback did not see the
cancellation.
handshakeCheck runs lock-free, so between the ctx check in periodicHandshakeCheck
and acquiring conn.mu a fast disconnect/reconnect can slip in: the stale watcher
then acquires the lock and tears down the *new*, healthy connection based on the
old timeout, forcing the guard into an unnecessary reconnect (flap).
Recheck watcherCtx.Err() under conn.mu so a superseded watcher exits without
touching the connection that replaced it.
ec98c930cb