Phase 5: async IO rendering — components call IO primitives client-side

Wire async rendering into client-side routing: pages whose component
trees reference IO primitives (highlight, current-user, etc.) now
render client-side via Promise-aware asyncRenderToDom. IO calls proxy
through /sx/io/<name> endpoint, which falls back to page helpers.

- Add has-io flag to page registry entries (helpers.py)
- Remove IO purity filter — include IO-dependent components in bundles
- Extend try-client-route with 4 paths: pure, data, IO, data+IO
- Convert tryAsyncEvalContent to callback style, add platform mapping
- IO proxy falls back to page helpers (highlight works via proxy)
- Demo page: /isomorphism/async-io with inline highlight calls

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/sx/ref/boot.sx

@@ -344,6 +344,7 @@
       (log-info (str "sx-browser " SX_VERSION))
       (init-css-tracking)
       (init-style-dict)
+      (init-io-primitives)
       (process-page-scripts)
       (process-sx-scripts nil)
       (sx-hydrate-elements nil)