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/conformance.sh

@@ -13,7 +13,7 @@ if [ ! -x "$SX_SERVER" ]; then
   exit 1
 fi
 
-SUITES=(event log kv project subscribe concurrency snapshot compaction durable)
+SUITES=(event log kv project subscribe concurrency snapshot compaction durable blob)
 
 OUT_JSON="lib/persist/scoreboard.json"
 OUT_MD="lib/persist/scoreboard.md"
@@ -36,6 +36,7 @@ run_suite() {
 (load "lib/persist/snapshot.sx")
 (load "lib/persist/compaction.sx")
 (load "lib/persist/durable.sx")
+(load "lib/persist/blob.sx")
 (load "lib/persist/subscribe.sx")
 (load "lib/persist/api.sx")
 (epoch 2)