A standing "no worse than the previous engine" check is not added to CI: the differential check already fails on unintended changes, and a fixed acceptance threshold would need renegotiating with every deliberate change. Instead, the rebuild procedure now runs the comparison while the frozen engine is still the previous one, and records it with the change. The runner compares against the frozen engine by default and takes `calibration` to reproduce the #302 record. Part of #302. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>