`BlobStats` carried a count and a byte total; it now carries a `BlobTally` per lifecycle state — held, referenced, unreferenced, collectable, and the two readings startup takes of the disk. `bot.did.stats` serves them under one `blobs` object, so the next blob condition worth reporting is a member of it rather than another top-level key. Breaking: `blobs` on that response is an object where it was a number, and `blobBytes` is now `blobs.held.bytes`. The write-ahead log is untouched, so `layout::SHAPE` is unchanged and a data directory needs nothing. Change-Id: I6d786f7ae36865a221a05045877d047e15e9f0f1