Measure synth repo persona diversity

Determine how different repo personas are. The ideal thing would be to measure the compression upper bound. If two repos are exactly the same, you should be able to compress to exactly the size of one of the repos.

The issue names no files, tests, or entry points. Start by locating the repository's synthetic-repo or persona measurement code, then define how compression should represent diversity. Done means the measure handles identical repositories by reaching the size of one repository and produces a meaningful comparison for different repositories.

Written by the indexing model from the issue text.