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>

package.json

@@ -17,9 +17,10 @@
     "data:fixtures": "bun scripts/buildFixtures.ts",
     "data:ratings": "bun scripts/buildRatings.ts",
     "data:preview": "bun scripts/buildPreviewData.ts",
+    "data:backtest": "bun scripts/buildBacktest.ts",
     "data:viz": "bun scripts/buildStatsbombViz.ts",
     "data:icons": "bun scripts/make-icons.mjs",
-    "data:build": "bun run data:icons && bun run data:fixtures && bun run data:ratings && bun run data:preview"
+    "data:build": "bun run data:icons && bun run data:fixtures && bun run data:ratings && bun run data:preview && bun run data:backtest"
   },
   "dependencies": {
     "@tanstack/react-router": "^1.95.0",