cmux ssh: X11 forwarding works but cmux shells have no DISPLAY

Symptom: after adding ForwardX11 yes for a host, GUI apps in cmux ssh shells fail with Cant open display (DISPLAY empty), while plain ssh -X works.

Cause: cmux-tui runs a long-lived headless daemon on the remote that spawns shells; the newest SSH link (cmux-tui-<name> remote-link, child of sshd-session) does get an X11 channel and DISPLAY=localhost:N.0 in its own environment, but the daemon started earlier and does not pass it to new shells.

Fix: in ~/.bashrc, when CMUX_TUI_TERMINAL_ID is set and DISPLAY is empty, read DISPLAY from /proc/<pid>/environ of the newest remote-link process (ps -u $USER --sort=-start_time, match args remote-link). Rerun after reconnects since N can change. Reading the port via ss -ltnp does not work without root (sshd-session fds are not readable). Avoid pgrep -f with the pattern in the same command line: it matches your own shell.