Add two read methods to PaperRepo for the A3f equity compounding work:
nav_equity_before returns the most recent equity strictly before a given
run_date; latest_nav_equity returns the most recent equity overall.
Both mirror the nav_history query idiom (select PaperNavRow, ordered by
run_date desc, limit 1). TDD: test written first, confirmed red, then green.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
07a3841065