The root exec's refusals before a line runs name the pane, and say invalid, so a mount reads EINVAL

A line over 1024 bytes or with a control character was refused with its
errno EINVAL but words 9ns reads as EIO (no `invalid` in them), and logged
`err - exec`. They now say `invalid command line: ...`, and the root's
err record names the pane the line would have run at.

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

1 files changed, 5 insertions, 2 deletions

diff --git a/docs/fs.md b/docs/fs.md
index 7470bdf3..bcf5dd5b 100644
--- a/docs/fs.md
+++ b/docs/fs.md
@@ -390,8 +390,11 @@ same tree without leaving the process.
   the next command there too. The log says `run <serial> <line>` and `exit
   <serial> <N|?>`; `exec` reads back the command pane's serial; Kill stops
   its whole line, `&` jobs included; a command line is at most 1024 bytes, and a
-  longer one written to an exec fails the write (EINVAL, `a command line is
-  at most 1024 bytes`) before anything in it runs; a builtin's line (a long
+  longer one written to an exec fails the write (EINVAL, `invalid command
+  line: a command line is at most 1024 bytes`, and one with a control
+  character but a tab, `invalid command line: it holds a control character
+  other than a tab`; the root's exec logs either against the pane it would
+  have run at) before anything in it runs; a builtin's line (a long
   `Msg`, an Edit block) may be longer. To run a command again, execute
   its line again from its directory: `echo 'make test' > pane/<n>/exec` on
   the command pane runs it there, below the last run. A misspelled word is a