Expand known components server-side in _aser to fix nested highlight calls

_aser previously serialized all ~component calls for client rendering.
Components whose bodies call Python-only functions (e.g. highlight) would
fail on the client with "Undefined symbol". Now _aser expands components
that are defined in the env via _aser_component, producing SX wire format
with tag-level bodies inlined. Unknown components still serialize as-is.

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

shared/sx/async_eval.py

@@ -1159,12 +1159,14 @@ async def _aser(expr: Any, env: dict[str, Any], ctx: RequestContext) -> Any:
         if name.startswith("html:"):
             return await _aser_call(name[5:], expr[1:], env, ctx)
 
-        # Component call — expand macros, serialize regular components
+        # Component call — expand macros, expand known components, serialize unknown
         if name.startswith("~"):
             val = env.get(name)
             if isinstance(val, Macro):
                 expanded = _expand_macro(val, expr[1:], env)
                 return await _aser(expanded, env, ctx)
+            if isinstance(val, Component):
+                return await _aser_component(val, expr[1:], env, ctx)
             return await _aser_call(name, expr[1:], env, ctx)
 
         # Serialize-mode special/HO forms (checked BEFORE HTML_TAGS