Make pty/run and /log streams, and take refused lines back cleanly

Review fixes to pty/run and /log:

- Both are streams with a per-open cursor. A shell's exec 3<>file shares
  one offset between write and read, so cat <&3 after echo make >&3 asked
  for offset 5 and got "0" instead of "exit 0"; log after follow lost its
  first bytes the same way.
- A run is accepted only in pardes's own tagged input phase, so a nested
  shell's prompt (ssh, a shell with its own integration) is never taken for
  this shell's.
- A line the shell refused is taken back so the next finds the prompt
  clear: bash's continuation prompt (now tagged k=c) gets Ctrl-C and the
  answer waits for the fresh prompt; a bash syntax error (no C, but a D) is
  answered at once; fish's kept line gets Ctrl-U (a Ctrl-C sent while fish
  redraws is lost). fish's right prompt no longer reads as typed input.
- bash marks a command's start from PS0 on bash 4.4+, not a DEBUG trap, so
  a user's own DEBUG trap (bash-preexec, atuin) survives. bash's own job
  notices now show as in plain bash (ttyfollow golden).
- 32 run slots; a new line on an open starts a fresh answer.

Checked live in bash, fish, and fish with a right prompt.

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

2 files changed, 16 insertions, 13 deletions

diff --git a/test/fs.py b/test/fs.py
index ff085a55..d44f417d 100644
--- a/test/fs.py
+++ b/test/fs.py
@@ -311,6 +311,9 @@ def run_file(binary):
                 time.sleep(.05)
             assert run(client, term, b'false\n') == b'exit 1\n'
             assert run(client, term, b'sh -c "exit 7"\n') == b'exit 7\n'
+            assert run(client, term, b'fi\n') == b'error not run\n'  # a syntax error runs nothing
+            assert run(client, term, b"echo 'open\n") == b'error not run\n'  # nor does an open quote
+            assert run(client, term, b'true\n') == b'exit 0\n'  # ...and the shell is clear after
             with Client(address) as other:
                 slow = []
                 waiter = threading.Thread(target=lambda: slow.append(run(other, term, b'sleep 1\n')), daemon=True)
diff --git a/test/snapshots/ttyfollow.golden b/test/snapshots/ttyfollow.golden
index e77b3caa..77f27d8e 100644
--- a/test/snapshots/ttyfollow.golden
+++ b/test/snapshots/ttyfollow.golden
@@ -28,17 +28,17 @@
 |8:   190                                               $
 == snap tty-follows grid=100x31 cursor=4,16
 |2: $  /tmp/pardes-snap/ttyfollow/cwd Tty Save Mode Fi$  /tmp/pardes-snap/ttyfollow/cwd Tty Save Mode Fi
-|3:   288                                               $ ls
-|4:   289                                               s.txt
-|5:   290                                               $ touch go; until test -f done1; do sleep 0.05;
-|6:   291                                               done; echo SYN''CED
-|7:   292                                               SYNCED
-|8:   293                                               $
-|9:   294
-|10:   295
-|11:   296
-|12:   297
-|13:   298
-|14:   299
-|15:   300
+|3:   292                                               $ ls
+|4:   293                                               s.txt
+|5:   294                                               $ touch go; until test -f done1; do sleep 0.05;
+|6:   295                                               done; echo SYN''CED
+|7:   296                                               SYNCED
+|8:   297                                               $
+|9:   298
+|10:   299
+|11:   300
+|12:   [1]+  Done                       ( until test -f
+|13:    go; do
+|14:       sleep 0.05;
+|15:   done; seq 201 260; touch done1 )
 |16:   $