fix(tui): terminal escapes ride the writer queue and teardown is bounded

A terminal that stopped draining the pty parked the writer thread inside
its tty write while it held the stderr lock; the next inline stderr write
from the event loop (mouse toggle, notification, kitty clear) deadlocked
the loop, and /quit hung on the unbounded writer join and the teardown
writes.

EscapeWriter queues out-of-band escapes on the frame writer: the mouse
capture toggle, notifications (title, progress, protocol), the auth
cursor, gboom keyboard flags, the wrap clipboard request, and the kitty
clears (queued per agent view and merged into the frame's post-flush,
nested subagent views included). The $EDITOR/$PAGER handoff aborts when
the writer does not drain. WriterThread::join_within and the teardown
run under a 2 s grace; after a stall fd 2 is silenced, the exit hints,
relaunch and update prints are skipped, and the panic hook keeps only its
log report.

Ported from upstream 1.0.19 (grok-build EscapeWriter, WriterJoin,
run_bounded_teardown), extended to the remaining inline writes.

Claude-Session: https://claude.ai/code/session_01RUqV8HVGXcAETyYbZ2tPQ4

crates/codegen/kigi-tty-utils/src/lib.rs

@@ -585,6 +585,19 @@ pub fn dup_tui_stderr() -> io::Result<std::fs::File> {
     }
 }
 
+/// Point fd 2 at `/dev/null` for the rest of the process: the terminal stopped reading, so any further stderr write would block.
+pub fn silence_native_stderr() {
+    #[cfg(unix)]
+    {
+        use std::os::unix::io::AsRawFd;
+        let Ok(devnull) = std::fs::OpenOptions::new().write(true).open("/dev/null") else {
+            return;
+        };
+        // SAFETY: devnull is a valid descriptor; fd 2 is a valid target.
+        unsafe { libc::dup2(devnull.as_raw_fd(), 2) };
+    }
+}
+
 /// Restore fd 2 to point to the real terminal stderr.
 ///
 /// Call this before exiting so that any final messages (panic output,