fix: use stored timestamps for paper dates instead of file mtime
Deploy copies all PDFs, clobbering their mtime to today. Now
metadata.json stores an `updated` ISO timestamp per paper, set only
when the paper is actually rebuilt. The index reads this instead of
stat().mtime, so dates survive full publishes and oven auto-builds.
Also backfills correct updated dates from git history and adds holden
paper metadata.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>