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/tui_gateway/test_protocol.py

@@ -613,44 +613,6 @@ def test_session_resume_live_payload_uses_current_history_with_ancestors(server,
     ]
 
 
-def test_session_activate_rebinds_orphaned_ws_session_to_current_transport(server, monkeypatch):
-    """Reconnect + activate must reattach a parked live session before orphan reap."""
-
-    class _Transport:
-        def write(self, _obj):
-            return True
-
-    sid = "runtime01"
-    old_transport = server._stdio_transport
-    new_transport = _Transport()
-    server._sessions[sid] = {
-        "agent": types.SimpleNamespace(model="test/model"),
-        "created_at": 123.0,
-        "history": [],
-        "history_lock": threading.RLock(),
-        "last_active": 123.0,
-        "running": False,
-        "session_key": "20260409_010101_abc123",
-        "transport": old_transport,
-    }
-    monkeypatch.setattr(server, "current_transport", lambda: new_transport)
-    monkeypatch.setattr(server, "_get_db", lambda: None)
-    monkeypatch.setattr(
-        server,
-        "_session_info",
-        lambda _agent, _session=None: {"model": "test/model"},
-    )
-
-    resp = server.handle_request(
-        {"id": "activate", "method": "session.activate", "params": {"session_id": sid}}
-    )
-
-    assert "error" not in resp
-    assert resp["result"]["session_id"] == sid
-    assert server._sessions[sid]["transport"] is new_transport
-    assert not server._ws_session_is_orphaned(server._sessions[sid])
-
-
 def test_session_branch_persists_branched_from_marker(server, monkeypatch):
     """TUI /branch must persist a _branched_from marker so the branch stays
     visible in /resume and /sessions.