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/helpers.py

@@ -691,6 +691,15 @@ def _build_pages_sx(service: str) -> str:
             deps = components_needed(content_src, _COMPONENT_ENV)
         deps_sx = "(" + " ".join(_sx_literal(d) for d in sorted(deps)) + ")"
 
+        # Check if any dep component has IO refs (needs async rendering)
+        from .types import Component as _Comp
+        has_io = "false"
+        for dep_name in deps:
+            comp = _COMPONENT_ENV.get(dep_name)
+            if isinstance(comp, _Comp) and comp.io_refs:
+                has_io = "true"
+                break
+
         # Build closure as SX dict
         closure_parts: list[str] = []
         for k, v in page_def.closure.items():
@@ -703,6 +712,7 @@ def _build_pages_sx(service: str) -> str:
             + " :path " + _sx_literal(page_def.path)
             + " :auth " + _sx_literal(auth)
             + " :has-data " + has_data
+            + " :has-io " + has_io
             + " :content " + _sx_literal(content_src)
             + " :deps " + deps_sx
             + " :closure " + closure_sx + "}"