Recency-weighted v4 model: fast-Elo blend, Team-DC majority, nightly refit

Validated on the strict 2018-2022 window and confirmed on the untouched
2022-2026 test set (RPS 0.1703 vs 0.1721 over 4,448 matches):
- the Elo member now blends 30% of a 3x-faster Elo walk, so recent
  results move ratings much harder
- ensemble weight shifts from 75/25 toward Elo to 45/55 toward the
  time-decayed Team-DC member — the recent-form model now leads
- Team-DC refits nightly (and at boot) on the 15-year window plus every
  finished 2026 match, via a new committed-at-build dcTrain.json
  (server-only, excluded from the PWA precache)
- a recent-form goal multiplier was also tested and did NOT validate;
  it ships disabled, and backtest.json records the whole decision

buildBacktest.ts grew the experiment harness: wider xi/k grids, the
fast-Elo and form variants, a pre-registered ship bar (>=0.0005
validation RPS), and a 7-day refit-cadence check. Older ratings.json
files still work — all new model fields are optional with golden
regression coverage.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.gitignore

@@ -19,6 +19,7 @@ public/data/ratings.json
 public/data/h2h.json
 public/data/scorers.json
 public/data/backtest.json
+public/data/dcTrain.json
 public/pwa-192x192.png
 public/pwa-512x512.png
 public/favicon.svg