Tree walker collects I/O nodes (frag, query, action, current-user,
htmx-request?), dispatches them via asyncio.gather(), substitutes results,
and renders to HTML. Failed I/O degrades gracefully to empty string.
27 new tests (199 total), all mocked at execute_io boundary — no
infrastructure dependencies needed.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
