fix(ui): prevent Dart event loop freezing by switching master PTY fd to non-blocking mode via fcntl

.agents/skills/multi-agent-mux-ui/packages/mam_pty/lib/src/pty_session.dart

@@ -50,6 +50,9 @@ typedef _waitpid_dart = int Function(int pid, ffi.Pointer<ffi.Int32> status, int
 typedef _chdir_c = ffi.Int32 Function(ffi.Pointer<ffi.Char> path);
 typedef _chdir_dart = int Function(ffi.Pointer<ffi.Char> path);
 
+typedef _fcntl_c = ffi.Int32 Function(ffi.Int32 fd, ffi.Int32 cmd, ffi.Int32 arg);
+typedef _fcntl_dart = int Function(int fd, int cmd, int arg);
+
 base class Winsize extends ffi.Struct {
   @ffi.Uint16()
   external int ws_row;
@@ -97,6 +100,7 @@ class PtySession {
     final execvp = libc.lookupFunction<_execvp_c, _execvp_dart>('execvp');
     final ioctl = libc.lookupFunction<_ioctl_c, _ioctl_dart>('ioctl');
     final chdir = libc.lookupFunction<_chdir_c, _chdir_dart>('chdir');
+    final fcntl = libc.lookupFunction<_fcntl_c, _fcntl_dart>('fcntl');
 
     // 1. Prepare master PTY
     final masterFd = posixOpenpt(2 | 0x00000400);
@@ -121,6 +125,13 @@ class PtySession {
     }
     final slaveName = slavePtr.cast<Utf8>().toDartString();
 
+    // Set master fd to non-blocking mode to prevent freezing the Dart event loop (§6.7)
+    // F_GETFL = 3, F_SETFL = 4, O_NONBLOCK = 2048 (0x800)
+    final flags = fcntl(masterFd, 3, 0);
+    if (flags >= 0) {
+      fcntl(masterFd, 4, flags | 2048);
+    }
+
     // 2. Resolve environment, isolating nested TMUX contexts (§6.7)
     final env = Map<String, String>.from(environment ?? Platform.environment);
     env.remove('TMUX');