fix(ui): implement async-signal-safe fork process layout and waitpid child zombie reaping for PTY

.agents/skills/multi-agent-mux-ui/packages/mam_pty/test/pty_runtime_test.dart

@@ -1,6 +1,5 @@
 import 'dart:async';
 import 'dart:convert';
-import 'dart:io';
 import 'package:mam_pty/mam_pty.dart';
 import 'package:test/test.dart';