persist: optimistic concurrency — conflict as a real result + 8 tests

concurrency.sx: persist/append-expect refuses an append when the stream
advanced past the caller's expected seq, returning {:conflict :expected
:actual} instead of crashing or overwriting. persist/conflict? + accessors.
Phase 2 complete, 54/54.

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)
+SUITES=(event log kv project subscribe concurrency)
 
 OUT_JSON="lib/persist/scoreboard.json"
 OUT_MD="lib/persist/scoreboard.md"
@@ -32,6 +32,7 @@ run_suite() {
 (load "lib/persist/log.sx")
 (load "lib/persist/kv.sx")
 (load "lib/persist/project.sx")
+(load "lib/persist/concurrency.sx")
 (load "lib/persist/subscribe.sx")
 (load "lib/persist/api.sx")
 (epoch 2)