tmux pipe-pane on every pane of the party session, appending into
<party dir>/log (0640, party group) so any attendee can grab a copy.
Start and stop are announced to everyone attached; close rescues a
non-empty transcript to the host's home before removing the dir.
Panes opened later are picked up by re-running party log (a
pane_pipe check keeps already-piped panes single-piped, since tmux's
pipe-pane -o toggles an existing pipe closed rather than skipping it).
Raw output, escapes included. Close polls the transcript for size
stability before rescuing it, since a cross-filesystem mv is
copy+unlink and could otherwise drop bytes still draining from the
pipe-pane writers.
