_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>