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/agent/test_memory_session_switch.py

@@ -179,7 +179,6 @@ def test_sync_all_propagates_session_id_to_providers():
     p = _RecordingProvider()
     mm.add_provider(p)
     mm.sync_all("hello", "world", session_id="sess-42")
-    mm.flush_pending(timeout=5)
     assert p.sync_calls == [
         {"user": "hello", "asst": "world", "session_id": "sess-42"}
     ]
@@ -190,7 +189,6 @@ def test_queue_prefetch_all_propagates_session_id_to_providers():
     p = _RecordingProvider()
     mm.add_provider(p)
     mm.queue_prefetch_all("next query", session_id="sess-42")
-    mm.flush_pending(timeout=5)
     assert p.queue_calls == [{"query": "next query", "session_id": "sess-42"}]