close the PTY master fd when the child exits naturally (T-360)

_reap() flipped _dead without releasing the master fd, and close()
short-circuits on _dead — so every naturally-exited child leaked its
fd and pty device for the life of the app. The reader isolate sends
EOF only after leaving its poll loop, so releasing the fd inside
_reap() cannot race the reader. Regression test counts /dev/ptmx
entries in /proc/self/fd across a natural exit; verified to fail
against the unfixed code.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

lib/src/pty/native_pty.dart

@@ -444,6 +444,11 @@ class NativePty {
   void _reap() {
     if (_dead) return;
     _dead = true;
+    // The reader isolate sends EOF only after exiting its poll loop, so
+    // nothing touches the master fd anymore. Release it here — close()
+    // short-circuits on _dead, so skipping this leaks the fd and its pty
+    // device for the life of the app on every natural child exit (T-360).
+    _nativeClose(_fd);
     final s = calloc<ffi.Int32>();
     _waitpid(pid, s, _kWnohang);
     calloc.free(s);