v2 Phase 3: rigorous backtest + Methodology page

- scripts/buildBacktest.ts: honest walk-forward validation — params fit on
  pre-2018 internationals, tested out-of-sample on 7,988 matches (2018-2026)
  predicting each game from prior data only. Proper scoring (Brier/log-loss/RPS/
  accuracy) vs uniform / base-rate / Elo-only baselines + a calibration analysis
  (reliability bins + ECE). Results: 60% accuracy, RPS 0.171 (beats all
  baselines), ECE 0.01 (excellent calibration).
- Methodology page (/methodology): plain-language model walkthrough, the backtest
  scorecard, a custom-SVG reliability diagram, and honest limits. Transparency is
  the differentiator — no market odds, no overclaiming.
- ReliabilityDiagram component; 'Model' nav entry.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitignore

@@ -16,6 +16,7 @@ public/data/fixtures.json
 public/data/ratings.json
 public/data/h2h.json
 public/data/scorers.json
+public/data/backtest.json
 public/pwa-192x192.png
 public/pwa-512x512.png
 public/favicon.svg