A desktop session holds 64 panes, and every route that finds none free says so

Sixteen panes was far too few for a real session. MAX_PANES is now 64 on a
desktop and stays 16 on the board; the 48 more slots cost about 3.7 KB each
in fixed tables (the core 1.33 to 1.45 MB, its surface 19 to 72 KB), and a
pane itself (84 KB and its terminal) is allocated only when it opens. At the
cap, pane/new fails with `pane/new: no pane slots: 64 max` and an err
record rather than a bare ENFILE, a 9P look or exec that would open a pane
fails its write with the same words and reads back empty, and New, Tty,
Tutor and Alt-n say it on the message row (their ctl writes fail with it).

Co-Authored-By: Claude Opus 5.5 <[email protected]>

1 files changed, 4 insertions, 1 deletions

diff --git a/docs/fs.md b/docs/fs.md
index fd18d523..55660bc0 100644
--- a/docs/fs.md
+++ b/docs/fs.md
@@ -109,7 +109,10 @@ Existing Plan9port/v9fs clients need a userspace bridge for QUIC.
              empty|full pane-serials...; then active <serial> (- when no column has the keyboard)
 /tag         the workspace tag; > replaces it, >> appends, one line
 /col/<n>/tag the tag of the column with serial n, the same way
-/pane/new    open it to make a pane; the read answers that pane's serial
+/pane/new    open it to make a pane; the read answers that pane's serial. A session holds
+             64 panes (16 on the board); at that, every route that would open one -- this
+             open, look, exec, New, Tty -- fails with `no pane slots: 64 max` and an err
+             record, and look reads back empty
 /pane/<n>/   name body tag ctl addr dot limit data xdata sel dirty mark scroll
              errors event look exec, plus pty/{ctl,status,data} on terminals
 /os/         the host filesystem