Nested component calls in _aser are serialized without body expansion,
so free variables inside ~root-header would be sent unresolved to the
client. Fix by making ~root-header/~root-mobile take all values as
&key params, and having parent layout defcomps pass them explicitly.
The parent layout bodies ARE expanded (via async_eval_slot_to_sx),
so their free variables resolve correctly during that expansion.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>