chore: untrack generated data; add momentum + fundamentals experiment

- Stop tracking data/sp500_history.json (rebuilt daily from Wikipedia by
  universe_history.load_sp500_history) and ignore data/books/ (per-machine
  live book state, like data/trader_*.json).
- research/lab/momentum_fundamentals.py: financial-statement vetoes and
  confirmations on overnight_recovery_top10 (2011-16 design, 2017-26
  holdout). No return gain; re-sorting the momentum top 30 by fundamental
  momentum lowers MaxDD at equal return.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

.gitignore

@@ -18,9 +18,11 @@ data/*.png
 
 # Universe caches — re-fetched daily
 data/universe_*.json
+data/sp500_history.json
 
 # Trader state — per-machine, regenerated by auto/simulate
 data/trader_*.json
+data/books/
 
 # Factor attribution output and cached factors
 data/attribution_*/