# Cup26 v5 — 20 features + a stronger model pipeline

Drafted after the v4 upgrade (i18n/DE, recency model, freshness, mobile). Each item
ships independently, smallest-risk-first within its phase; the app stays deployable
after every commit. Hard constraints stand: bookmaker odds are benchmark-only, squad
values stay out of the model, no auto-anything that pretends to be betting advice.

## Model improvement: better than a single validation window

Today: params fit pre-2018 → hyperparams tuned on 2018–2022 → tested once on
2022–2026; the shipped model then trains on ALL data with those hyperparams.
The weakness is hyperparam selection on ONE window — it can overfit that
window's quirks (e.g. the grid-floor ξ result).

**v5 scheme — rolling-origin cross-validation (walk-forward CV):**
1. Five rolling validation folds: tune-eval on 2014–16, 2016–18, 2018–20,
   2020–22, 2022–24, each predicting strictly walk-forward with params fit
   only on data before the fold. Pick hyperparams by MEAN RPS across folds
   (ties → fewer parameters), not by a single window.
2. Keep 2024–2026 as the untouched FINAL test, evaluated once, reported
   honestly in backtest.json (the public Methodology page).
3. Add a final probability-calibration layer: isotonic regression fit
   walk-forward on the CV folds' predictions, applied at runtime, validated
   the same way (ship only if mean-fold RPS/ECE improve).
4. Widen what gets tuned (same discipline): HOME_ADV_ELO, the Elo K scale,
   stage-conditional ensemble weight (group vs knockout), Team-DC window
   length. All cheap grid axes on cached walks.
5. Shipped model still trains through today + nightly Team-DC refit + live
   fast-Elo re-rating (v4) — recency at runtime, selection by rolling CV.

## The 20 features

### A. Live & match experience
1. **In-play win-probability timeline** — server stores the in-play prob each
   poll while live; match page draws the momentum curve (the "story of the
   match" in one chart).
2. **Odds-movement chart per match** — odds_history is already captured every
   3h; plot the de-vigged market line vs our frozen model number from
   prediction_snapshots.
3. **Goal/kickoff notifications (opt-in)** — PWA Web Push: pick matches or a
   team, get goals + kickoff pings (server already knows score changes).
4. **Penalty-shootout live view** — ESPN summary carries shootout events;
   render kick-by-kick during shootouts instead of the generic timeline.
5. **Upset meter** — after each match, how surprising was the result vs the
   frozen model (RPS percentile); a tournament-wide "biggest shocks" list.

### B. Predict & explore
6. **Bracket what-if mode** — tap winners through the knockout tree; recompute
   title odds client-side under your picks (matchMatrix runs in the browser).
7. **Group qualification scenarios** — "X advances if…" engine over the final
   group matchday (exhaustive enumeration of remaining group results).
8. **Team comparison view** — two teams side-by-side: Elo trajectory, form,
   style fingerprint (StatsBomb), squad value, H2H, model matchup.
9. **Elo history charts** — precompute each WC team's rating path (1872→now,
   and tournament-zoom) at data:build; sparkline on team pages.
10. **H2H explorer** — pick any two nations (search), full 150-year record +
    the model's hypothetical matchup.
11. **Golden Boot race** — live top-scorer table from ESPN events + historical
    scorer context per player.
12. **Model diary** — public log of nightly refits and biggest rating moves
    ("Morocco +14 after beating Spain"), feeding the transparency story.

### C. App & UX
13. **Favorite team** — star a team: pinned next-match card on Live, its odds
    evolution, one-tap to its page (localStorage, no accounts).
14. **Daily digest view** — one screen: yesterday's results + model movement,
    today's matches with predictions, the title-odds delta.
15. **Global search / command palette** — Cmd-K + mobile search: jump to any
    team or match.
16. **Timezone-correct schedule everywhere** + "my local time" chips and a
    week-strip calendar view of all 104 fixtures.
17. **Offline polish** — cache the last snapshot in the SW; show a "stale —
    last updated X" banner instead of empty states when offline.
18. **Accessibility pass** — focus rings, aria-labels on all probability bars
    (text alternatives with real numbers), prefers-reduced-motion, contrast
    audit in both themes.
19. **Performance pass** — route-level code-splitting (recharts already lazy),
    hover/viewport prefetch, list virtualization where >50 cards, font subset.
20. **Share links** — per-match/per-team canonical URLs with server-rendered
    OG meta (title + the model's numbers) so shared links unfurl nicely.

## Order

Model pipeline first (it gates everything math-adjacent: 1, 5, 6, 12), then
B6/B7 (pure client math, high wow), then A1/A2 (data already on disk), then
C13–C16, then the rest. Verify per item: tests + build gate + preview matrix
(EN/DE × mobile/desktop × themes); deploy in batches of 2-3 features.
