A write applies the keys it changed to the repository already built for that head and re-signs it, instead of building the whole repository from the records; one that finds nothing held builds as before. A seeded test compares every write against a build from the records, its `since` diff and its firehose frame, across a restart. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com> Change-Id: I50364b1f6c457dafc0f01cbb000d9c07e455b3ad