kvserver: 16 byte discrepancy in `SysBytes`

**Describe the problem**

We've seen a 16 byte discrepancy in KVNemesis runs here: https://github.com/cockroachdb/cockroach/issues/159331. See comments: [[1]](https://github.com/cockroachdb/cockroach/issues/159331#issuecomment-3647717679) [[2]](https://github.com/cockroachdb/cockroach/issues/159331#issuecomment-3648049083) for my inconclusive analysis thus far.

We should figure out the source of this discrepancy and fix it. This is intended to be a placeholder issue we can link and close subsequent KVNemesis failures to, if they arise, lest we miss more critical failures in the noise.

Jira issue: CRDB-57836

Start with the KVNemesis failure and the 16-byte SysBytes discrepancy described here, then read the linked issue and its referenced comments for the existing analysis. Trace the source of the discrepancy in the kvserver accounting path. Done means the cause is identified and the discrepancy is fixed so subsequent KVNemesis failures do not reproduce it.

Written by the indexing model from the issue text.