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/honcho_plugin/test_client.py

@@ -819,15 +819,10 @@ class TestResolveSessionNameLengthLimit:
 class TestResetHonchoClient:
     def test_reset_clears_singleton(self):
         import plugins.memory.honcho.client as mod
-
-        # Seed the cached client through the slot's public surface, then
-        # verify reset_honcho_client() clears it. (The client is cached in
-        # mod._honcho_client_slot, a thread-safe SingletonSlot, not a bare
-        # module global anymore — see #24759.)
-        mod._honcho_client_slot.get(lambda: MagicMock())
-        assert mod._honcho_client_slot.peek() is not None
+        mod._honcho_client = MagicMock()
+        assert mod._honcho_client is not None
         reset_honcho_client()
-        assert mod._honcho_client_slot.peek() is None
+        assert mod._honcho_client is None
 
 
 class TestDialecticDepthParsing: