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_gateway.py

@@ -369,16 +369,6 @@ def test_systemd_install_checks_linger_status(monkeypatch, tmp_path, capsys):
     unit_path = tmp_path / "systemd" / "user" / "hermes-gateway.service"
 
     monkeypatch.setattr(gateway, "get_systemd_unit_path", lambda system=False: unit_path)
-    # Synthetic unit with a non-temp home: the real generator bakes the
-    # hermetic test HERMES_HOME (a tmp dir), which the temp-home write
-    # guard correctly refuses.
-    monkeypatch.setattr(
-        gateway,
-        "generate_systemd_unit",
-        lambda system=False, run_as_user=None: (
-            '[Service]\nEnvironment="HERMES_HOME=/home/alice/.hermes"\n'
-        ),
-    )
 
     calls = []
     helper_calls = []
@@ -406,15 +396,6 @@ def test_systemd_install_can_skip_enable_on_startup(monkeypatch, tmp_path, capsy
     unit_path = tmp_path / "systemd" / "user" / "hermes-gateway.service"
 
     monkeypatch.setattr(gateway, "get_systemd_unit_path", lambda system=False: unit_path)
-    # Non-temp home so the temp-home write guard (which trips on the
-    # hermetic test HERMES_HOME) stays out of the way.
-    monkeypatch.setattr(
-        gateway,
-        "generate_systemd_unit",
-        lambda system=False, run_as_user=None: (
-            '[Service]\nEnvironment="HERMES_HOME=/home/alice/.hermes"\n'
-        ),
-    )
 
     calls = []
     helper_calls = []