opentui(phase3): launcher integration — HERMES_TUI_ENGINE dual-engine

hermes --tui launches the native OpenTUI engine (Bun) when
HERMES_TUI_ENGINE=opentui (env) or display.tui_engine=opentui (config);
Ink stays the default and the shipping path is untouched.

- _resolve_tui_engine() (env > config > ink); refuses opentui on
  Windows/Termux (no Bun) -> falls back to ink with a notice.
- _make_opentui_argv() -> [bun, src/entry.real.tsx] (no build step).
- _bun_bin() with HERMES_BUN override.
- Branch at top of _make_tui_argv BEFORE _ensure_tui_node (Bun-only host
  must not bootstrap Node).
- Gate _launch_tui NODE_OPTIONS/--max-old-space-size on engine==ink (Bun
  is JSC; the V8 flag errors/ignores).

Verified end-to-end via tmux: real hermes --tui -> Bun -> OpenTUI ->
real Python gateway streamed a real reply. No-flag default still ink.

tools/process_registry.py

@@ -472,7 +472,6 @@ class ProcessRegistry:
                     text=True,
                     timeout=10,
                     creationflags=windows_hide_flags(),
-                    stdin=subprocess.DEVNULL,
                 )
             except (FileNotFoundError, subprocess.TimeoutExpired, OSError):
                 try:
@@ -1208,14 +1207,10 @@ class ProcessRegistry:
         if session.exited:
             return {"status": "already_exited", "error": "Process has already finished"}
 
-        # PTY mode -- write through pty handle.
+        # PTY mode -- write through pty handle (expects bytes)
         if hasattr(session, '_pty') and session._pty:
             try:
-                # 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
+                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: