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.md

@@ -9,4 +9,5 @@ _Generated by `lib/persist/conformance.sh`_
 | kv | 13 | 0 | 13 |
 | project | 9 | 0 | 9 |
 | subscribe | 9 | 0 | 9 |
-| **Total** | **46** | **0** | **46** |
+| concurrency | 8 | 0 | 8 |
+| **Total** | **54** | **0** | **54** |