Add _system/brain-format.json { dataFormat, indexEpoch } + a sync brain.formatInfo()
accessor + a compiled EXPECTED_INDEX_EPOCH, the surface a native provider reads at
init() to drive whole-brain auto-upgrade, and brainy's own derived-index
rebuild-on-format-drift trigger (closing the gap where JS indexes rebuilt only on
size()===0, never on a format-version change).
indexEpoch is shared and lockstep-bumped with the native provider on any coordinated
release whose on-disk derived-index format changes; dataFormat is brainy-owned. On open,
the marker is read in the store-open phase BEFORE provider construction (so formatInfo()
is synchronously available at the provider's init); a drifted or absent epoch rebuilds
the derived indexes from canonical records, and the marker is stamped only AFTER the
rebuild verifies (build-new -> verify -> stamp; a crash before the stamp re-triggers the
idempotent rebuild). brainFormat.ts is the single source of the shared constants.
6-case test; 1724 unit green.
