Closes the crash-window resurrection corner in the #5532 clear-conversation
data-loss fix. /api/session/clear writes a .json.bak before truncating, and
because the cleared sidecar's truncation_watermark is 0.0, startup recovery saw
bak_count > live_count and could restore the backup — resurrecting the cleared
transcript after a crash/restart in the clear window.
Fix: stamp a unique per-clear `clear_generation` UUID on the sidecar when
clearing a session that had messages (persisted via Session model). Recovery's
_session_records_clear_sentinel() returns no_action when the live sidecar carries
a clear_generation the backup doesn't share AND the live sidecar matches the
exact cleared-empty shape (messages/context empty, watermark/boundary 0.0,
pending fields cleared). Same-generation backups stay recoverable; unreadable or
partial matches FAIL OPEN (normal recovery), so a real crash-loss is never
suppressed.
Contributor stage; gate-pass. Co-authored-by: rodboev <rodboev@users.noreply.github.com>
