`SESSION_FORMAT_VERSION` is the only value that names the Session format
generation. Two other version fields had moved to 2 alongside it:
- The `dsh_session_log` request extension's outer schema `version` returns
to 1 and its Session header projection keeps `seedLength`, derived from
the logical inherited cut, so the external wire is unchanged in this PR;
`sessionFormatVersion` alone identifies the embedded generation.
- The `sessionStats` projection unit's `stateVersion` returns to 1: the
projection cache binds every checkpoint to the format generation, so a
generation change discards old rows without a unit version bump.
Test doubles and the standalone Client fixture that build a current
header now spell `SESSION_FORMAT_VERSION` instead of the literal.
_Kerman