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.json

@@ -8,9 +8,10 @@
     "concurrency": {"pass": 8, "fail": 0},
     "snapshot": {"pass": 11, "fail": 0},
     "compaction": {"pass": 11, "fail": 0},
-    "durable": {"pass": 15, "fail": 0}
+    "durable": {"pass": 15, "fail": 0},
+    "blob": {"pass": 14, "fail": 0}
   },
-  "total_pass": 91,
+  "total_pass": 105,
   "total_fail": 0,
-  "total": 91
+  "total": 105
 }