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/sxc/pages/docs.sx

@@ -444,6 +444,17 @@
     :server-time server-time :items items
     :phase phase :transport transport))
 
+(defpage async-io-demo
+  :path "/isomorphism/async-io"
+  :auth :public
+  :layout (:sx-section
+    :section "Isomorphism"
+    :sub-label "Isomorphism"
+    :sub-href "/isomorphism/"
+    :sub-nav (~section-nav :items isomorphism-nav-items :current "Async IO")
+    :selected "Async IO")
+  :content (~async-io-demo-content))
+
 ;; Wildcard must come AFTER specific routes (first-match routing)
 (defpage isomorphism-page
   :path "/isomorphism/<slug>"