Persist provider_data as a {producer, blocks} envelope (producer = the generating
provider's name) so the lowering layer can later replay the native lane verbatim only
to its producer and rebuild from neutral fields for any other.
The envelope is storage-only: prepare_provider_data_for_save runs the P1 mirror on the
bare block list and then wraps; reconstruct_messages dual-reads (new envelope OR legacy
bare list), unwraps to a bare _provider_content list (every consumer's contract), and
surfaces the producer on the stripped-before-wire _producer side channel. The producer
threads the same four save layers as is_error (facade -> protocol -> SQLite + Postgres);
the live assistant save tags it from self._provider.provider_name and the fork carries
_producer. Legacy rows need no migration to keep working (dual-read); the one-shot
backfill that tags them is a follow-up.
Sub-commit 2a of the canonical-trajectory storage cut.
Patrick Buckley