A command pane's command is over when its process exits, not when its pty closes

A job left in the background (sleep 100 &) held the pty open, so the pane
stayed running and the child a zombie until the job ended; a command that
closed its terminal and ran on got its end of file at once, the host waited
100 ms for an exit, reported exit ? and hung it up. Now each command's child
is watched on a thread (waitid with WNOWAIT, so its pid stays its own until
the host reaps it), and the host tells the core the exit from that: after the
pty's end of file, so the output before the exit is in, or 50 ms after the
exit without one, a job holding the pty. The pty stays open until both, so a
command that let go of its terminal is never hung up by it. All four front
ends; a host that cannot start the watcher reads the exit at end of file as
before. Tests: host_io's for both cases, and cmdexit.snap end to end.

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

1 files changed, 14 insertions, 0 deletions

diff --git a/test/snapshots/cmdexit.golden b/test/snapshots/cmdexit.golden
new file mode 100644
index 00000000..58412752
--- /dev/null
+++ b/test/snapshots/cmdexit.golden
@@ -0,0 +1,14 @@
+== snap opened grid=120x31 cursor=7,3
+|Newcol Joincol Find Grep Help Changelog Tutor Dump NextColor Debug Exit
+|   New Tty Find Grep Joincol Delcol
+|   /tmp/pardes-snap/cmdexit/cwd/cmds.txt Save Tty Collapse Del
+|     1 sleep 30 & echo BG''STARTED
+|     2 exec </dev/null >/dev/null 2>&1; sleep 1; false
+== snap background grid=120x31 cursor=7,3
+|17:    /tmp/pardes-snap/cmdexit/cwd (sleep 30 & echo BG''STARTED) exit 0 Kill Save Collapse Del
+|18:   BGSTARTED
+|19:   exit 0
+== snap letgo grid=120x31 cursor=7,3
+|17:    /tmp/pardes-snap/cmdexit/cwd (exec </dev/null >/dev/null 2>&1; sleep 1; false) exit 1 Kill Save Collapse Del
+|20:   % exec </dev/null >/dev/null 2>&1; sleep 1; false
+|21:   exit 1