v0.3.13

Remove per-batch overlay_ref guard, now redundant with watermark

The truncated_before watermark is written before any deletions and is
checked by create_namespace inside its transaction. This makes the
per-batch non-snapshot read of overlay_ref unnecessary — there is no
window in which a fork can be created against already-truncated data.

Removing it eliminates a non-snapshot range read (and potential conflict
and retry) from every deletion batch transaction.

Also bump all crates to v0.3.13.