persist: Phase 1 — log + kv facets on injectable in-memory backend + 28 tests

event/backend/log/kv/api over one injected backend protocol (mem default).
log: append/read/read-from, sequential per-stream seq, stream isolation.
kv: get/put/delete/has?/keys/get-or/update. conformance.sh + 3 suites, 28/28.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/persist/scoreboard.md

@@ -0,0 +1,10 @@
+# persist Conformance Scoreboard
+
+_Generated by `lib/persist/conformance.sh`_
+
+| Suite | Pass | Fail | Total |
+|-------|-----:|-----:|------:|
+| event | 6 | 0 | 6 |
+| log | 9 | 0 | 9 |
+| kv | 13 | 0 | 13 |
+| **Total** | **28** | **0** | **28** |