persist: durable backend over the perform IO boundary + 15 tests

durable.sx: io-backend with an injectable transport — persist/durable-backend
performs each op as {:op "persist/..." :args (...)} (kernel suspends, host
resumes); persist/mock-durable services via persist/serve over an in-memory
disk. Identical request shapes mean the whole facet/projection/snapshot/
compaction stack runs unchanged on the durable backend. Crash/restart replay
recovers log+kv+snapshot. 91/91.

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

lib/persist/scoreboard.json

@@ -7,9 +7,10 @@
     "subscribe": {"pass": 9, "fail": 0},
     "concurrency": {"pass": 8, "fail": 0},
     "snapshot": {"pass": 11, "fail": 0},
-    "compaction": {"pass": 11, "fail": 0}
+    "compaction": {"pass": 11, "fail": 0},
+    "durable": {"pass": 15, "fail": 0}
   },
-  "total_pass": 76,
+  "total_pass": 91,
   "total_fail": 0,
-  "total": 76
+  "total": 91
 }