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/scoreboard.json

@@ -4,9 +4,10 @@
     "log": {"pass": 9, "fail": 0},
     "kv": {"pass": 13, "fail": 0},
     "project": {"pass": 9, "fail": 0},
-    "subscribe": {"pass": 9, "fail": 0}
+    "subscribe": {"pass": 9, "fail": 0},
+    "concurrency": {"pass": 8, "fail": 0}
   },
-  "total_pass": 46,
+  "total_pass": 54,
   "total_fail": 0,
-  "total": 46
+  "total": 54
 }