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.md

@@ -12,4 +12,5 @@ _Generated by `lib/persist/conformance.sh`_
 | concurrency | 8 | 0 | 8 |
 | snapshot | 11 | 0 | 11 |
 | compaction | 11 | 0 | 11 |
-| **Total** | **76** | **0** | **76** |
+| durable | 15 | 0 | 15 |
+| **Total** | **91** | **0** | **91** |