persist: blob backend — store the ref/CID, never the bytes + 14 tests

blob.sx: a blob ref is {:cid :size :mime}; the blob store is a separate
injected dependency (perform in prod, mock content store in tests).
persist/blob-store puts bytes and returns only the ref; bytes live in a
content-addressed store (artdag/IPFS). Tests assert refs in log/kv never carry
the bytes + content-address dedup. 105/105.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/persist/scoreboard.md

@@ -13,4 +13,5 @@ _Generated by `lib/persist/conformance.sh`_
 | snapshot | 11 | 0 | 11 |
 | compaction | 11 | 0 | 11 |
 | durable | 15 | 0 | 15 |
-| **Total** | **91** | **0** | **91** |
+| blob | 14 | 0 | 14 |
+| **Total** | **105** | **0** | **105** |