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/conformance.sh

@@ -13,7 +13,7 @@ if [ ! -x "$SX_SERVER" ]; then
   exit 1
 fi
 
-SUITES=(event log kv project subscribe concurrency snapshot compaction)
+SUITES=(event log kv project subscribe concurrency snapshot compaction durable)
 
 OUT_JSON="lib/persist/scoreboard.json"
 OUT_MD="lib/persist/scoreboard.md"
@@ -35,6 +35,7 @@ run_suite() {
 (load "lib/persist/concurrency.sx")
 (load "lib/persist/snapshot.sx")
 (load "lib/persist/compaction.sx")
+(load "lib/persist/durable.sx")
 (load "lib/persist/subscribe.sx")
 (load "lib/persist/api.sx")
 (epoch 2)