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/gateway/test_update_command.py

@@ -86,15 +86,12 @@ class TestHandleUpdateCommand:
             class FakePath(type(Path())):
                 pass
 
-            # Actually, simplest: just patch the specific file attr.
-            # The _handle_update_command handler lives in gateway/slash_commands.py
-            # (extracted from run.py in the god-file decomposition); it resolves
-            # project_root via Path(__file__).parent.parent, so fake that file.
-            fake_file = str(fake_root / "gateway" / "slash_commands.py")
+            # Actually, simplest: just patch the specific file attr
+            fake_file = str(fake_root / "gateway" / "run.py")
             (fake_root / "gateway").mkdir(parents=True)
-            (fake_root / "gateway" / "slash_commands.py").touch()
+            (fake_root / "gateway" / "run.py").touch()
 
-            with patch("gateway.slash_commands.__file__", fake_file):
+            with patch("gateway.run.__file__", fake_file):
                 result = await runner._handle_update_command(event)
 
         assert "Not a git repository" in result