S3 (a1): affiliate + partner plumbing

Zero out-of-pocket; everything config-flip-ready but DISABLED/organic.

- BOOK IT deep links: web/src/lib/bookLinks.js + affiliateConfig.js
  (all books enabled:false, Impact/Partnerize param shapes documented,
  empty params skipped). Wired into StatStrip BookItTeaser (real anchor
  now) + scan hand-off links. Every book anchor renders
  rel="sponsored noopener noreferrer" (BOOK_LINK_REL).
- Best-price marker: slateAdapter.detectBestBook (only when >=2 books
  post the SAME line and prices differ — absent beats wrong) + subtle
  signal-green dot in StatStrip. Slate.groupByGame threads the grouped
  per-book rows (books[]) onto PropRowProp instead of discarding them.
- Partner refs: ?ref=CODE -> vyndr_ref cookie (90d, first-touch,
  PartnerRefCapture in layout) -> signup metadata partner_ref ->
  internal GET /api/partners/report/:code (requireInternalAuth; honest
  zeros + note until the TODO migration in docs/PARTNERS.md adds
  user_profiles.partner_ref — NOT run). Stripe promo-code convention:
  partner code == promotion code, verbatim.
- Tests: +41 (2398 -> 2439, 209 suites); bookItTeaser + vyndrCoreScreens
  invariants updated to the new (stronger) rel contract. Web build exit 0.

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

web/src/lib/slateAdapter.js

@@ -51,6 +51,49 @@ function detectBestLines(books) {
   }));
 }
 
+/**
+ * Best available price across a prop's book rows (A1 Session 3).
+ *
+ * `rows` is the grouped odds shape the Express /api/odds proxy already
+ * ships the browser: [{ book, line, over_odds, under_odds }] — one row
+ * per book for the same player+stat.
+ *
+ * Data-semantics rule: a "best price" claim is only honest when ≥2 books
+ * post the SAME line for the side and their prices differ — comparing
+ * odds across different lines is meaningless, and a lone price isn't
+ * "best". Anything else → null. Absent beats wrong.
+ *
+ * @param {Array<{book?:string,line?:number,over_odds?:number|null,under_odds?:number|null}>} rows
+ * @param {string} [side] — 'over' | 'under' (default over)
+ * @param {number|null} [refLine] — the line the card displays; when given,
+ *   only books at that exact line compete.
+ * @returns {{ book: string, odds: number } | null}
+ */
+function detectBestBook(rows, side = 'over', refLine = null) {
+  const key = String(side || 'over').toLowerCase().startsWith('u') ? 'under_odds' : 'over_odds';
+  const valid = (Array.isArray(rows) ? rows : []).filter(
+    (r) => r && r.book && Number.isFinite(r.line) && r[key] != null && parseAmericanOdds(r[key]) != null,
+  );
+  if (valid.length < 2) return null;
+
+  // Compare at ONE line: the displayed line when given, else the modal line.
+  let line = Number.isFinite(refLine) ? refLine : null;
+  if (line == null) {
+    const counts = new Map();
+    for (const r of valid) counts.set(r.line, (counts.get(r.line) || 0) + 1);
+    let bestCount = 0;
+    for (const [ln, c] of counts) if (c > bestCount) { bestCount = c; line = ln; }
+  }
+  const atLine = valid.filter((r) => r.line === line);
+  if (atLine.length < 2) return null;
+
+  const decimals = atLine.map((r) => parseAmericanOdds(r[key]));
+  const max = Math.max(...decimals);
+  if (max === Math.min(...decimals)) return null; // identical prices → no "best"
+  const winner = atLine[decimals.indexOf(max)];
+  return { book: winner.book, odds: winner[key] };
+}
+
 function formatGameTime(iso) {
   if (!iso) return '';
   try {
@@ -309,6 +352,11 @@ function buildPlayerStripsFromProps(gameProps, gradeIndex, deltaIndex, now = Dat
         line: rec.line,
         side,
         grade: rec.grade,
+        // A1 S3 — the prop's own book + the best available price across the
+        // game's book rows for the graded side (null unless ≥2 books at the
+        // same current line disagree — see detectBestBook).
+        book: p.book || null,
+        bestBook: detectBestBook(p.books, side === 'U' ? 'under' : 'over', p.line),
         gradedAt: rec.gradedAt
           ? { ...rec.gradedAt, ago: gradedAgo(rec.gradedAt.timestamp, now) }
           : null,
@@ -324,6 +372,8 @@ function buildPlayerStripsFromProps(gameProps, gradeIndex, deltaIndex, now = Dat
     } else {
       byPlayer[pk].props.push({
         stat: statShort(p.stat_type || p.stat), line: p.line, side: '', grade: null, awaiting: true,
+        book: p.book || null,
+        bestBook: detectBestBook(p.books, p.direction || 'over', p.line),
       });
     }
   }
@@ -374,6 +424,7 @@ function pitchersForGameTeams(awayTeam, homeTeam, pitcherMap) {
 module.exports = {
   parseAmericanOdds,
   detectBestLines,
+  detectBestBook,
   mapGameLines,
   mapScheduleToGameCards,
   formatGameTime,