Homepage participation graph: card-denominated ratio, headline flip, in-session green dot

[WilfordGrimley]

Summary

• Moves ParticipationGraph's gate + drawn bar off the votes-over-cards approximation (humanVotes.total / total) onto the exact card-denominated ratio distinctCardsRoutedToReviewWithHumanVotes / distinctCardsRoutedToReview (PR catalog-stats: add card-denominated participation counts #566's new fields). Old path and its units caveat deleted entirely, not kept as a fallback.
• humanProgressRatioPercent now returns null (never NaN) when the three card-denominated fields aren't all present, finite numbers - the guaranteed state of every 1/catalogStats/ response until the production API redeploys past catalog-stats: add card-denominated participation counts #566. shouldRevealHumanProgress treats null as unconditionally below-threshold, so the below-threshold design renders byte-for-byte unchanged during that window.
• Revealed headline flips to a "cards the machine routed to people" framing (distinct copy, not a bar bolted onto the below-threshold sentences). Not latched - orchestration issue VoteSource AI->DEDUCTION/OCR split + local OCR/phash printing-ID pilot (Stage 8) #22 tracks that as a known, accepted limitation.
• New ReviewedCardCount renders distinctCardsRoutedToReviewWithHumanVotes as a plain, always-rising count beside the bar, since distinctCardsRoutedToReview (the ratio's denominator) only ever grows.
• Dot-matrix's dashed "you would be the Nth" mark becomes a filled green dot + short thank-you once this browser tab has cast a vote in-session, via a new sessionContributionSlice.ts (in-memory Redux state only - no localStorage, no new endpoint).

Deviations from the stated file scope

The in-session vote-tracking requirement ("drive it from in-session state... via the existing Redux/RTK Query wiring") has no existing cross-page wiring to read from: QuestionFeed.tsx's vote-casting calls are plain fetch helpers in store/api.ts, not RTK Query mutations, and session state there is local useState, not Redux. Satisfying the requirement as written needed two small, additive touches outside the nominally listed scope:

• frontend/src/store/store.ts - register the new sessionContribution reducer (2 lines).
• frontend/src/features/questionFeed/QuestionFeed.tsx - bumpSessionCount (the existing single choke point every successful vote in that feed already calls) now also dispatches recordSessionContribution() (6 lines).

Both are additive only, no existing behavior changed; full frontend test suite (Jest + Playwright) passes including QuestionFeed/WhatsThat specs.

Test plan

• npx next build
• npx tsc --noEmit
• npx jest - 73 suites / 666 tests passed
• npx playwright test --project=chromium - 335 passed, 6 skipped, 0 failed
• python3 .github/scripts/docs_lint.py --strict - clean
• python3 .github/scripts/coverage_delta.py --base 80814167 - clean

🤖 Generated with Claude Code

https://claude.ai/code/session_01Nz54BtscqFNzeKuDzEcpT3