RAR cache dirty reason should be logged

RAR should log the reason why the state file became dirty and why it chose to write it. It's hard to diagnose why the state file is written every time on empty rebuilds where nothing changes.

Locate the RAR cache logic that marks the state file dirty and decides to write it, then inspect how empty rebuilds reach that path. Reproduce an empty rebuild and identify the existing diagnostics around the decision. Done means the logs explain why the state became dirty and why the file was written.

Written by the indexing model from the issue text.