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>

src/lib/types.ts

@@ -160,6 +160,22 @@ export interface TeamProfile {
   qualifyOdds: number | null;
 }
 
+// ---- model backtest (Methodology page) ----
+
+export interface BacktestScores { brier: number; logloss: number; rps: number; accuracy: number }
+export interface BacktestReport {
+  generatedAt: string;
+  trainEnd: string;
+  testTo: string;
+  tested: number;
+  params: { goalsPerElo: number; avgGoals: number; rho: number; homeAdvElo: number };
+  model: BacktestScores;
+  baselines: { uniform: BacktestScores; baseRate: BacktestScores; eloOnly: BacktestScores };
+  reliability: { predicted: number; observed: number; count: number }[];
+  ece: number;
+  worldCup: { tested: number; accuracy: number };
+}
+
 // ---- data-story viz (StatsBomb open data, a fixed historical match) ----
 
 export interface VizShot {