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.

tests/tools/test_process_registry.py

@@ -63,44 +63,6 @@ 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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