pty/run takes one line a run, its answer read before the next

Two lines -- in one write, or as bash writes them, a line at a time on one open -- each ran, and the first's answer was lost under the second's. A write with more than one line, or a next line before the last answer is read, is now refused, EINVAL.

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

1 files changed, 3 insertions, 1 deletions

diff --git a/docs/fs.md b/docs/fs.md
index 6bf30212..de1113bd 100644
--- a/docs/fs.md
+++ b/docs/fs.md
@@ -967,7 +967,9 @@ acme's window keeps one `eventx`; a second read on it meanwhile fails with
 `pty/run` runs one line at a terminal's prompt and answers how it ended, on
 the same open (factotum's `rpc` shape): write the line, then read `exit N`
 once the command has ended and the shell is back at a prompt, followed by
-what it printed. The output is what the screen showed between the command's
+what it printed. One line a run: two lines in one write, or a next line on
+the open before the last answer is read (bash writes `printf 'a\nb\n'` a
+line at a time), are refused, EINVAL. The output is what the screen showed between the command's
 start and end marks: stderr interleaved, `\r` progress collapsed to its last
 state, no colour, tabs as the spaces they drew, trailing spaces trimmed and
 trailing blank lines dropped (`printf 'a\n\n\n'` answers `a`), leading