v1 blob transport: the blob rides the :peer payload on dist, admitted
only under ctx.max_blob_size (default 65_536 — implementation.md §9),
measured as :erlang.external_size/1, the actual dist-payload proxy. The
cap is what keeps "dist carries control only" true as a Key-layer
discipline (implementation.md §7): per-key granularity already bounds
messages to one key's state; this bounds one key's state.