Cheap rewind

Build lab (Module 10, L10.5): content-addressed checkpoints of 4 files, manifest written last. Rollback is byte-identical 4/4 and the stray is gone (1). The same-size corruption is 35 bytes: a size check passes it 1 time and sha256 catches it 1 time. A crash after the manifest is written first is complete 0 with the manifest present 1. A crash before the manifest is written last is complete 0 with the manifest present 0. A fresh checkpoint ALLOWs a refactor and keeps a webhook at ASK. Allowing everything once a checkpoint exists lets the webhook through. In-memory tree plus a scratch directory that is deleted; no bucket, threads, model or provider call.

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