f30074c4ae
Content-addressed repository on an IBackend (local implementation): 4 MiB blobs, deflate when it helps, each blob encrypted with its id as associated data, ~32 MiB packs with an encrypted header, one index file per pack, snapshots as encrypted JSON lines written last. Spike B: 390 MiB tree, peak 54 MiB; unchanged run reads nothing; a 1 MiB change in a 300 MiB file uploads one blob; full and partial restore are bit-identical; tampering is detected.