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/pages.py

@@ -331,6 +331,9 @@ def auto_mount_pages(app: Any, service_name: str) -> None:
     if has_data_pages:
         auto_mount_page_data(app, service_name)
 
+    # Mount IO proxy endpoint for Phase 5: client-side IO primitives
+    mount_io_endpoint(app, service_name)
+
 
 def mount_pages(bp: Any, service_name: str,
                 names: set[str] | list[str] | None = None) -> None:
@@ -535,3 +538,73 @@ def auto_mount_page_data(app: Any, service_name: str) -> None:
         methods=["GET"],
     )
     logger.info("Mounted page data endpoint for %s at /sx/data/<page_name>", service_name)
+
+
+def mount_io_endpoint(app: Any, service_name: str) -> None:
+    """Mount /sx/io/<name> endpoint for client-side IO primitive calls.
+
+    The client can call any allowed IO primitive or page helper via GET/POST.
+    Result is returned as SX wire format (text/sx).
+
+    Falls back to page helpers when the name isn't a global IO primitive,
+    so service-specific functions like ``highlight`` work via the proxy.
+    """
+    import asyncio as _asyncio
+    from quart import make_response, request, abort as quart_abort
+    from .primitives_io import IO_PRIMITIVES, execute_io
+    from .jinja_bridge import _get_request_context
+    from .parser import serialize
+
+    # Allowlist of IO primitives + page helpers the client may call
+    _ALLOWED_IO = {
+        "highlight", "current-user", "request-arg", "request-path",
+        "htmx-request?", "app-url", "asset-url", "config",
+    }
+
+    async def io_proxy(name: str) -> Any:
+        if name not in _ALLOWED_IO:
+            quart_abort(403)
+
+        # Parse args from query string or JSON body
+        args: list = []
+        kwargs: dict = {}
+        if request.method == "GET":
+            for k, v in request.args.items():
+                if k.startswith("_arg"):
+                    args.append(v)
+                else:
+                    kwargs[k] = v
+        else:
+            data = await request.get_json(silent=True) or {}
+            args = data.get("args", [])
+            kwargs = data.get("kwargs", {})
+
+        # Try global IO primitives first
+        if name in IO_PRIMITIVES:
+            ctx = _get_request_context()
+            result = await execute_io(name, args, kwargs, ctx)
+        else:
+            # Fall back to page helpers (service-specific functions)
+            helpers = get_page_helpers(service_name)
+            helper_fn = helpers.get(name)
+            if helper_fn is None:
+                quart_abort(404)
+            result = helper_fn(*args, **kwargs) if kwargs else helper_fn(*args)
+            if _asyncio.iscoroutine(result):
+                result = await result
+
+        result_sx = serialize(result) if result is not None else "nil"
+        resp = await make_response(result_sx, 200)
+        resp.content_type = "text/sx; charset=utf-8"
+        return resp
+
+    io_proxy.__name__ = "sx_io_proxy"
+    io_proxy.__qualname__ = "sx_io_proxy"
+
+    app.add_url_rule(
+        "/sx/io/<name>",
+        endpoint="sx_io_proxy",
+        view_func=io_proxy,
+        methods=["GET", "POST"],
+    )
+    logger.info("Mounted IO proxy endpoint for %s at /sx/io/<name>", service_name)