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/gateway/test_matrix_exec_approval.py

@@ -27,9 +27,9 @@ class TestMatrixExecApprovalReactions:
         assert result.success is True
         assert adapter._approval_prompt_by_session["sess-1"] == "$evt1"
         assert adapter._approval_prompts_by_event["$evt1"].session_key == "sess-1"
-        assert adapter._send_reaction.await_count == 3
+        assert adapter._send_reaction.await_count == 2
         emojis = [call.args[2] for call in adapter._send_reaction.await_args_list]
-        assert emojis == ["✅", "♾️", "❌"]
+        assert emojis == ["✅", "❎"]
 
     @pytest.mark.asyncio
     async def test_reaction_resolves_pending_approval(self, monkeypatch):