Pardes control filesystem
=========================
README      this guide
index       one line per pane: serial, kind (text|term|pdf|image), dirty flag, name
status      pid, version and pane count
look        write a line: a right click on it at the active pane; read: the serials it touched
exec        write a line: a middle click, an editor command word or a shell line; read the same
log         recent events: new/del/rename/save <serial> <name>, msg <serial|-> <text>; write follow to wait
screen      rendered screen as JSON, frozen from open to close
listeners   the session's dial addresses
focus       the serial of the pane with the keyboard; write a serial to give it the keyboard
ctl         the settings, one a line as a write takes them; write one, or Newcol, Dump; Kill QUITS
commands    every builtin: its word, arg if it takes one, and root or pane: which ctl takes it
pane/new    open it to make a pane; the read answers that pane's serial
pane/<n>/   name body tag ctl addr dot limit data xdata sel dirty mark scroll
            errors event look exec, and pty/ for terminals; rmdir closes it
os/, src/   the host filesystem; the editor's own sources (-Dembed-sources=true builds only)

Below, $m is the mount point (PARDES_MOUNT in a Tty9p shell; 9ns and 9p work too):

  cat $m/index                                 which panes exist
  n=$(cat $m/pane/new)                        make a pane, take its serial
  echo /etc/hosts:3 > $m/look; cat $m/look     open a file, see the pane it went to
  printf 'text\n' > $m/pane/$n/body            append to a pane (>| truncates first)
  cat $m/pane/$n/name; echo notes.txt > $m/pane/$n/name        read, then rename
  echo Save > $m/pane/$n/exec                  save it; rmdir $m/pane/$n closes it
  echo 'Msg hello' > $m/exec                   show text in the editor
  echo '#0,#5' > $m/pane/$n/addr; printf NEW >> $m/pane/$n/data  replace bytes 0..5
  cp $m/pane/$n/addr $m/pane/$n/dot; cat $m/pane/$n/sel   select the range, read it
  exec 3<>$m/log; echo follow >&3; cat <&3     what happened lately, then wait for more
  cat $m/pane/$n/event                         hold it open to own that pane's clicks
  echo exec > $m/pane/$n/pty/ctl               restart a shell; also winsize C R, sig INT
  exec 3<>$m/pane/$n/pty/run; echo make >&3; cat <&3   exit N then its output; or busy, or error
  printf 'ls\r' > $m/pane/$n/pty/data           type into a terminal; pty/status: cols rows busy

Pitfalls, one each:
  Each open of pane/new makes another pane (ls, stat, find never do); a reread names the same.
  rmdir closes a pane even when it is dirty; index and pane/<n>/dirty show the flag.
  addr, dot and limit read the same pair of offsets they take, so cp between them works.
  dirty, mark and scroll read "0" or "1" and take "0" or "1"; truncating limit lifts it.
  body: >> appends, > replaces it all. data: > replaces (and : > deletes) just the addr range.
  tag reads path++text; writes append to the text (newlines too); truncating clears it, and dot or addr.
  A terminal's body is a history snapshot frozen per open; pty/data is the live stream.
  A failing command is reported in the editor and in log, not as a write error; a bad line fails the write.
  pane/<n>/ctl: acme's status line + current; takes get, lock/unlock (on a held fd) and pane builtins (Del).