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]>

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