fit_as_of does not pin a training set; training_cutoff does

The reconstruction verifier re-walked the ledger bounded by `fit_as_of` and
reported DIFFERS for a completely healthy artifact. The artifact had not moved —
its knot, curve and source digests on disk were byte-identical and the runtime
was serving exactly them. The TRAINING SOURCE had moved.

Measured: the artifact was built at 05:xx UTC with 6,069 rows through
2026-09-01. By 20:00 UTC a re-walk at the same fit_as_of returned 6,237, because
2026-09-02's 168 rows settled in between. Settlement backfills dates that are
already before the cutoff, so `fit_as_of` bounds eligibility but does not pin
membership.

`training_cutoff` — the newest observation actually admitted — does pin it,
because those dates are fully settled. Re-walked under that bound the
reconstruction is EXACT: fit_n, training_cutoff, source_digest, knot_count,
knot_digest, served_curve_digest all equal and the curve identical.

This is why the two fields were kept separate and why source_digest exists. The
digest caught a moving source and refused to call it a match, which is the
behaviour that made the diagnosis possible.

No artifact change. No model change. The verifier now states the bound it used.

Suite 405/405.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CQJeAG8vcDoL5zkiaJyVb8

scripts/verify-artifact-reconstruction.js

@@ -38,9 +38,23 @@ function servedCurve(map, bands) {
   const a = registry.load('mlb', 'hits');
   if (!a) throw new Error('no promoted artifact');
 
-  // ONLY what the artifact declares about its own training contract.
+  // BOUND BY `training_cutoff`, NOT `fit_as_of`.
+  //
+  // `fit_as_of` is the eligibility bound the ORIGINAL walk used, but it does not
+  // pin the training set: settlement backfills. Measured 2026-09-03 — the
+  // artifact was built at 05:xx UTC with 6,069 rows through 2026-09-01, and by
+  // 20:00 UTC a re-walk at the same `fit_as_of` returned 6,237, because
+  // 2026-09-02's 168 rows had settled in between. Reconstruction against
+  // `fit_as_of` therefore reports DIFFERS for a completely healthy artifact.
+  //
+  // `training_cutoff` is the newest observation actually admitted, and those
+  // dates are fully settled, so it is stable. `lt` is exclusive, so the bound is
+  // the day AFTER the cutoff.
+  const cutoffDay = new Date(`${a.training_cutoff}T00:00:00Z`);
+  cutoffDay.setUTCDate(cutoffDay.getUTCDate() + 1);
+  const bound = cutoffDay.toISOString().slice(0, 10);
   const rows = await src.loadRows(sb, {
-    sport: a.sport, stat: a.stat, modelVersion: a.model_version, before: a.fit_as_of,
+    sport: a.sport, stat: a.stat, modelVersion: a.model_version, before: bound,
   });
   rows.sort((x, y) => (x.date < y.date ? -1 : x.date > y.date ? 1 : (x.id < y.id ? -1 : 1)));
   const audit = src.eraAudit(rows, a.model_version);
@@ -62,7 +76,7 @@ function servedCurve(map, bands) {
     served_curve_digest: a.served_curve_digest, wrong_era_rows: 0,
   };
   const mismatches = Object.keys(want).filter((k) => got[k] !== want[k]);
-  console.log(JSON.stringify({ artifact_id: a.artifact_id, want, got,
+  console.log(JSON.stringify({ artifact_id: a.artifact_id, bound_used: `game_date < ${bound}`, want, got,
     curve_identical: JSON.stringify(curve) === JSON.stringify(a.served_curve),
     mismatches, RECONSTRUCTION: mismatches.length === 0 && JSON.stringify(curve) === JSON.stringify(a.served_curve)
       ? 'EXACT' : 'DIFFERS' }, null, 1));