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/kv.sx

@@ -0,0 +1,26 @@
+; persist/kv — the kv facet: current-state values, no history. For things
+; whose history does NOT matter (stock counts, config, profiles, session
+; blobs) and where projections materialize their read models.
+; Requires: lib/persist/backend.sx.
+
+(define persist/kv-get (fn (b key) (persist/backend-kv-get b key)))
+(define
+  persist/kv-put
+  (fn (b key val) (begin (persist/backend-kv-put b key val) val)))
+(define persist/kv-delete (fn (b key) (persist/backend-kv-delete b key)))
+(define persist/kv-has? (fn (b key) (persist/backend-kv-has? b key)))
+(define persist/kv-keys (fn (b) (persist/backend-kv-keys b)))
+
+; get with a default when the key is absent
+(define
+  persist/kv-get-or
+  (fn
+    (b key dflt)
+    (if (persist/kv-has? b key) (persist/kv-get b key) dflt)))
+
+; read-modify-write: apply f to the current value (or dflt if absent), store result
+(define
+  persist/kv-update
+  (fn
+    (b key dflt f)
+    (persist/kv-put b key (f (persist/kv-get-or b key dflt)))))