fix(tui): handle Windows PTY stdin and detached WS frames (#41953)

Two narrow Windows desktop fixes:

1. tools/process_registry.py — PTY stdin writes are now platform-aware.
   pywinpty (Windows) expects str; ptyprocess (POSIX) expects bytes.
   Previously bytes was unconditionally passed, producing a TypeError on
   Windows ("'bytes' object cannot be converted to 'PyString'").

2. tui_gateway/server.py + ws.py — Detached WebSocket sessions now park on
   a _DropTransport sink instead of _stdio_transport. In the desktop the
   gateway runs in-process and stdout is captured by Electron into
   desktop.log, so falling back to stdio leaked raw JSON-RPC frames into
   the desktop log after WS disconnects. Orphan-reap semantics are
   preserved via _ws_session_is_orphaned.

Verified on a Windows desktop install:
- pywinpty 2.0.15 rejects bytes / accepts str — reproduced exactly
- Focused suite green (write_stdin × 2, write_json_drops_detached_ws_frames,
  ws_orphan_reap × 2)
- All 6 CI test shards green, e2e green, nix (ubuntu/macos) green

Salvage commit (21be7ca) fixes the new test referencing an undefined
_ThreadUnsafeStdout — uses the existing _ChunkyStdout helper.

tests/tools/test_process_registry.py

@@ -63,6 +63,44 @@ def _wait_until(predicate, timeout: float = 5.0, interval: float = 0.05) -> bool
     return False
 
 
+def test_write_stdin_uses_str_for_windows_pty(monkeypatch, registry):
+    """pywinpty expects str input; bytes raises a PyString conversion error."""
+    written = []
+
+    class _FakePty:
+        def write(self, value):
+            written.append(value)
+
+    session = _make_session(sid="pty-win")
+    session._pty = _FakePty()
+    registry._running[session.id] = session
+    monkeypatch.setattr("tools.process_registry._IS_WINDOWS", True)
+
+    result = registry.write_stdin(session.id, "hello\n")
+
+    assert result == {"status": "ok", "bytes_written": 6}
+    assert written == ["hello\n"]
+    assert isinstance(written[0], str)
+
+
+def test_write_stdin_uses_bytes_for_posix_pty(monkeypatch, registry):
+    written = []
+
+    class _FakePty:
+        def write(self, value):
+            written.append(value)
+
+    session = _make_session(sid="pty-posix")
+    session._pty = _FakePty()
+    registry._running[session.id] = session
+    monkeypatch.setattr("tools.process_registry._IS_WINDOWS", False)
+
+    result = registry.write_stdin(session.id, "hello\n")
+
+    assert result == {"status": "ok", "bytes_written": 6}
+    assert written == [b"hello\n"]
+
+
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