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/tools/test_web_providers_ddgs.py

@@ -190,11 +190,7 @@ class TestDDGSBackendWiring:
         monkeypatch.setattr(web_tools, "_ddgs_package_importable", lambda: True)
         assert web_tools._get_backend() == "exa"
 
-    def test_auto_detect_prefers_keyless_parallel_over_ddgs(self, monkeypatch):
-        # With no credentials, keyless Parallel is the auto-detect default even
-        # when the ddgs package is installed — ddgs is search-only (can't
-        # extract), so Parallel is preferred so both search and extract work.
-        # ddgs remains reachable via an explicit web.backend=ddgs.
+    def test_auto_detect_picks_ddgs_as_last_resort(self, monkeypatch):
         from tools import web_tools
         monkeypatch.setattr(web_tools, "_load_web_config", lambda: {})
         for key in ("FIRECRAWL_API_KEY", "FIRECRAWL_API_URL", "PARALLEL_API_KEY",
@@ -202,7 +198,7 @@ class TestDDGSBackendWiring:
             monkeypatch.delenv(key, raising=False)
         monkeypatch.setattr(web_tools, "_is_tool_gateway_ready", lambda: False)
         monkeypatch.setattr(web_tools, "_ddgs_package_importable", lambda: True)
-        assert web_tools._get_backend() == "parallel"
+        assert web_tools._get_backend() == "ddgs"
 
     def test_check_web_api_key_true_when_ddgs_configured(self, monkeypatch):
         from tools import web_tools