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_computer_use.py

@@ -338,7 +338,7 @@ class TestCaptureResponse:
         from tools.computer_use.backend import CaptureResult
         from tools.computer_use import tool as cu_tool
 
-        fake_png = "iVBORw0KGgoAAAANSUhEUgAAAAgAAAAICAYAAADED76LAAAADUlEQVR4nGNgGAUgAAABCAABgukLHQAAAABJRU5ErkJggg=="
+        fake_png = "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAQAAAC1HAwCAAAAC0lEQVR42mNkYAAAAAYAAjCB0C8AAAAASUVORK5CYII="
 
         class FakeBackend:
             def start(self): pass
@@ -372,41 +372,11 @@ class TestCaptureResponse:
         assert any(p.get("type") == "image_url" for p in out["content"])
         assert any(p.get("type") == "text" for p in out["content"])
 
-    def test_capture_tiny_image_returns_text_json(self):
-        """Providers can reject <8px images, so placeholders must be omitted."""
-        from tools.computer_use.backend import CaptureResult, UIElement
-        from tools.computer_use import tool as cu_tool
-
-        tiny_png = "iVBORw0KGgoAAAANSUhEUgAAAAIAAAACCAYAAABytg0kAAAAC0lEQVR4nGNgQAcAABIAAXfx+gAAAAAASUVORK5CYII="
-
-        cap = CaptureResult(
-            mode="som",
-            width=0,
-            height=0,
-            png_b64=tiny_png,
-            elements=[
-                UIElement(index=1, role="AXButton", label="Continue", bounds=(10, 20, 30, 30)),
-            ],
-            app="Safari",
-            window_title="Example",
-            png_bytes_len=68,
-        )
-
-        with patch.object(cu_tool, "_should_route_through_aux_vision",
-                          return_value=False):
-            out = cu_tool._capture_response(cap)
-
-        parsed = json.loads(out)
-        assert parsed["width"] == 2
-        assert parsed["height"] == 2
-        assert "screenshot omitted" in parsed["summary"]
-        assert parsed["elements"][0]["label"] == "Continue"
-
     def test_capture_som_with_elements_formats_index(self):
         from tools.computer_use.backend import CaptureResult, UIElement
         from tools.computer_use import tool as cu_tool
 
-        fake_png = "iVBORw0KGgoAAAANSUhEUgAAAAgAAAAICAYAAADED76LAAAADUlEQVR4nGNgGAUgAAABCAABgukLHQAAAABJRU5ErkJggg=="
+        fake_png = "iVBORw0KGgo="
 
         class FakeBackend:
             def start(self): pass