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_session.py

@@ -611,30 +611,6 @@ class TestSessionStoreSwitchSession:
         db.close()
 
 
-class TestSessionStoreLookupBySessionId:
-    @pytest.fixture()
-    def store(self, tmp_path):
-        config = GatewayConfig()
-        with patch("gateway.session.SessionStore._ensure_loaded"):
-            s = SessionStore(sessions_dir=tmp_path, config=config)
-        s._db = None
-        s._loaded = True
-        return s
-
-    def test_returns_active_entry_for_persisted_session_id(self, store):
-        source = SessionSource(
-            platform=Platform.MATRIX,
-            chat_id="!room:example.org",
-            chat_type="group",
-            user_id="@alice:example.org",
-        )
-        entry = store.get_or_create_session(source)
-
-        assert store.lookup_by_session_id(entry.session_id) is entry
-        assert store.lookup_by_session_id("missing") is None
-        assert store.lookup_by_session_id("") is None
-
-
 class TestWhatsAppSessionKeyConsistency:
     """Regression: WhatsApp session keys must collapse JID/LID aliases to a
     single stable identity for both DM chat_ids and group participant_ids."""