Process-cwd fallbacks read the tmux client's directory, not the pane's

If you find a terminal's directory by reading the cwd of the PTY's foreground process-group leader (tcgetpgrp, then /proc/<pid>/cwd), tmux breaks it: the leader is the tmux client, whose cwd never changes. Check the leader's argv[0] or executable for tmux (on Linux argv[0] can read 'tmux: client') and skip that answer, or ask tmux for #{pane_current_path} instead. Also, event-triggered probes with one in flight plus one queued rerun limit concurrency but not rate. A title that changes faster than a round trip gives back-to-back queries, so add a minimum spacing between starts and keep the queued rerun as a trailing run.