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.

tools/process_registry.py

@@ -1207,10 +1207,14 @@ class ProcessRegistry:
         if session.exited:
             return {"status": "already_exited", "error": "Process has already finished"}
 
-        # PTY mode -- write through pty handle (expects bytes)
+        # PTY mode -- write through pty handle.
         if hasattr(session, '_pty') and session._pty:
             try:
-                pty_data = data.encode("utf-8") if isinstance(data, str) else data
+                # pywinpty expects str on Windows; ptyprocess expects bytes on POSIX.
+                if _IS_WINDOWS:
+                    pty_data = data.decode("utf-8") if isinstance(data, bytes) else str(data)
+                else:
+                    pty_data = data.encode("utf-8") if isinstance(data, str) else data
                 session._pty.write(pty_data)
                 return {"status": "ok", "bytes_written": len(data)}
             except Exception as e: