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>

sx/sx/nav-data.sx

@@ -107,7 +107,8 @@
   (dict :label "Roadmap" :href "/isomorphism/")
   (dict :label "Bundle Analyzer" :href "/isomorphism/bundle-analyzer")
   (dict :label "Routing Analyzer" :href "/isomorphism/routing-analyzer")
-  (dict :label "Data Test" :href "/isomorphism/data-test")))
+  (dict :label "Data Test" :href "/isomorphism/data-test")
+  (dict :label "Async IO" :href "/isomorphism/async-io")))
 
 (define plans-nav-items (list
   (dict :label "Reader Macros" :href "/plans/reader-macros"