test(planning): rebuild the frozen engine from the obligation ranking

The frozen engine of the differential check is rebuilt from 8168137,
which plans from the best-ranked obligation solution, with the
documented build command. The check passes again, including its
cancelled asynchronous comparisons and generator-coverage assertions.
The calibration engine stays at a871f8f.

Part of #302.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/benchmark/README.md

@@ -120,8 +120,11 @@ trace counters and timings are historical; current evidence is in
 
 `src/app/services/frozen-engine.spec.ts` runs 5,000 seeded random planning
 requests (seeds 280000 to 284999, one test per 200 seeds) through the current
-engine and through a frozen copy of the engine at `a871f8f`, develop after
-PR #278. For each request, the planning outcome, stable plan explanation,
+engine and through a frozen copy of the engine. The copy was rebuilt for #302
+from `8168137`, which plans from the best-ranked obligation solution
+([comparison record](issue-302.md#comparison-record)). Until then it was the
+engine at `a871f8f`, develop after PR #278, which remains the calibration
+engine. For each request, the planning outcome, stable plan explanation,
 explanation error and final-state snapshot must be strictly deep-equal,
 including `PlanningConflict` and `InvalidInput` outcomes. Timings are not
 compared.