checkpoint: chain shadow, WNBA possession feed, baseball chain

Backup commit of uncommitted working-tree state found during Legion
recon (Tony resurrection, STEP 0). This work existed only on the
laptop disk.

- chain shadow accrual + probe script (038_chain_shadow.sql)
- WNBA possession feed: ESPN adapter, usage service, verify script
  (039_wnba_player_game.sql)
- baseball chain
- retention/snapshot service updates, tableKeys, matchupKeys
- specs: chain-v1, wnba-possession-feed, wnba-source-survey
- unit tests for the above

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QnvJAkC3h5QGmb6dipoiWn

src/services/retentionService.js

@@ -156,6 +156,13 @@ function rowsFromSides(base, sides, ctx = {}) {
       // the multiplier is recomputable from them and is deliberately not stored.
       // Null on every non-hits row and on any row with no factor context.
       factor_inputs: s.factor_inputs || null,
+
+      // The SHADOW CHAIN read. Declared here (always present, usually null) so
+      // every row in a batch carries the same keys — PostgREST builds a bulk
+      // insert from the FIRST row's shape, so a column that appears only on some
+      // rows is silently dropped for the whole batch. Filled by
+      // `mergeChainShadow` after enrichment; never read by anything served.
+      chain_shadow: null,
     });
   }
   return rows;
@@ -246,6 +253,43 @@ function mergeEnrichment(rows, enrichedGrades) {
   });
 }
 
+/**
+ * Attach the SHADOW CHAIN read to collected rows — the (chain_p, counter_p,
+ * outcome) triple's first two thirds.
+ *
+ * Like `mergeEnrichment` this fills ONE field and touches nothing else. It runs
+ * later than grade time for the same structural reason the archetype does: the
+ * chain needs the statcast rows and park factors the enrichment pass loaded, and
+ * pulling that forward into the grader would put per-prop I/O on the serving
+ * path.
+ *
+ * THE SIDE ALIGNMENT IS THE LOAD-BEARING PART. The chain computes P(over the
+ * line); `p_win` on the row is expressed for the graded SIDE. Storing the raw
+ * over-probability against an under row's `p_win` would invert every comparison
+ * made from it afterwards, silently — so `alignToSide` is given the row's own
+ * side and the row's own counter probability, and both halves of the triple end
+ * up pointing the same way. `outcome` is written by the ordinary settle pass and
+ * is already side-aligned, which completes it.
+ *
+ * A row the chain could not read is left NULL. It is not a zero probability and
+ * not an average hitter — the chain refusing to read someone is a fact worth
+ * keeping, and a fabricated third of a triple would poison the adjudication this
+ * column exists to enable.
+ */
+function mergeChainShadow(rows, shadow) {
+  if (!Array.isArray(rows) || !rows.length) return rows || [];
+  const byKey = shadow && shadow.byKey;
+  if (!byKey || typeof byKey.get !== 'function') return rows;
+  const cs = require('./model/chainShadow');
+
+  return rows.map((r) => {
+    const block = byKey.get(cs.shadowKey(r.player_key, r.stat, r.line));
+    if (!block) return r;
+    const aligned = cs.alignToSide(block, r.side, r.p_win);
+    return aligned ? { ...r, chain_shadow: aligned } : r;
+  });
+}
+
 function newSnapshotId() {
   return crypto.randomUUID();
 }
@@ -257,6 +301,7 @@ module.exports = {
   rowsFromSides,
   createCollector,
   mergeEnrichment,
+  mergeChainShadow,
   persist,
   newSnapshotId,
   __internals: { numOrNull, intOrNull, boolOrNull },