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/hermes_cli/test_tui_npm_install.py

@@ -26,12 +26,6 @@ def _touch_tui_entry(root: Path) -> None:
     entry.write_text("console.log('tui')")
 
 
-def _assert_utf8_replace_capture(kwargs: dict) -> None:
-    assert kwargs["text"] is True
-    assert kwargs["encoding"] == "utf-8"
-    assert kwargs["errors"] == "replace"
-
-
 def test_need_install_when_ink_missing(tmp_path: Path, main_mod) -> None:
     (tmp_path / "package-lock.json").write_text("{}")
     assert main_mod._tui_need_npm_install(tmp_path) is True
@@ -234,8 +228,6 @@ def test_make_tui_argv_scopes_npm_install_on_termux_workspace(
         "--include-workspace-root=false",
     ]
     assert calls[0][1]["cwd"] == str(tmp_path)
-    _assert_utf8_replace_capture(calls[0][1])
-    _assert_utf8_replace_capture(calls[1][1])
 
 
 def test_make_tui_argv_keeps_desktop_workspace_install_behaviour(
@@ -271,8 +263,6 @@ def test_make_tui_argv_keeps_desktop_workspace_install_behaviour(
         "--progress=false",
     ]
     assert calls[0][1]["cwd"] == str(tmp_path)
-    _assert_utf8_replace_capture(calls[0][1])
-    _assert_utf8_replace_capture(calls[1][1])
 
 
 def test_make_tui_argv_keeps_desktop_always_build_behaviour(
@@ -296,35 +286,6 @@ def test_make_tui_argv_keeps_desktop_always_build_behaviour(
 
     assert calls
     assert calls[0][0][0] == ["/bin/npm", "run", "build"]
-    _assert_utf8_replace_capture(calls[0][1])
-
-
-def test_make_tui_argv_decodes_dev_prebuild_with_utf8_replace(
-    tmp_path: Path, main_mod, monkeypatch
-) -> None:
-    ink_dir = tmp_path / "packages" / "hermes-ink"
-    ink_dir.mkdir(parents=True)
-    tsx = tmp_path / "node_modules" / ".bin" / "tsx"
-    tsx.parent.mkdir(parents=True)
-    tsx.write_text("")
-
-    monkeypatch.setattr(main_mod, "_tui_need_npm_install", lambda _root: False)
-    monkeypatch.setattr(main_mod.shutil, "which", lambda name: f"/bin/{name}")
-    calls = []
-
-    def fake_run(*args, **kwargs):
-        calls.append((args, kwargs))
-        return types.SimpleNamespace(returncode=0, stdout="", stderr="")
-
-    monkeypatch.setattr(main_mod.subprocess, "run", fake_run)
-
-    argv, cwd = main_mod._make_tui_argv(tmp_path, tui_dev=True)
-
-    assert argv == [str(tsx), "src/entry.tsx"]
-    assert cwd == tmp_path
-    assert calls[0][0][0] == ["/bin/npm", "run", "build"]
-    assert calls[0][1]["cwd"] == str(ink_dir)
-    _assert_utf8_replace_capture(calls[0][1])
 
 
 # ── _workspace_root helper ──────────────────────────────────────────
@@ -464,43 +425,3 @@ def test_tui_launch_install_uses_workspace_scope(
     install_cmd = npm_calls[0]
     assert "--workspace" in install_cmd
     assert "ui-tui" in install_cmd
-
-def test_make_tui_argv_omits_workspace_when_tui_has_own_lockfile(
-    tmp_path: Path, main_mod, monkeypatch
-) -> None:
-    """When ui-tui/ has its own package-lock.json, _workspace_root returns
-    tui_dir itself.  npm install --workspace ui-tui would fail in that case
-    because npm cannot find a workspace named "ui-tui" inside ui-tui/.
-    The fix omits --workspace and runs plain npm install from tui_dir.
-    See #42973.
-    """
-    tui_dir = tmp_path / "ui-tui"
-    tui_dir.mkdir()
-    (tui_dir / "package.json").write_text("{}")
-    # Simulate curl-install layout: tui_dir has its own lockfile
-    (tui_dir / "package-lock.json").write_text("{}")
-    # Parent also has lockfile (but _workspace_root prefers tui_dir's own)
-    (tmp_path / "package-lock.json").write_text("{}")
-
-    monkeypatch.delenv("TERMUX_VERSION", raising=False)
-    monkeypatch.setenv("PREFIX", "/usr")
-    monkeypatch.setattr(main_mod, "_tui_need_npm_install", lambda _root: True)
-    monkeypatch.setattr(main_mod.shutil, "which", lambda name: f"/bin/{name}")
-    calls = []
-
-    def fake_run(*args, **kwargs):
-        calls.append((args, kwargs))
-        return types.SimpleNamespace(returncode=0, stdout="", stderr="")
-
-    monkeypatch.setattr(main_mod.subprocess, "run", fake_run)
-
-    main_mod._make_tui_argv(tui_dir, tui_dev=False)
-
-    install_cmd = calls[0][0][0]
-    # Must NOT contain --workspace when npm_cwd == tui_dir
-    assert "--workspace" not in install_cmd, (
-        f"npm install should omit --workspace when tui_dir has its own lockfile, got: {install_cmd}"
-    )
-    assert install_cmd[:2] == ["/bin/npm", "install"]
-    # cwd must be tui_dir (standalone), not parent
-    assert calls[0][1]["cwd"] == str(tui_dir)