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/backend.sx

@@ -0,0 +1,21 @@
+; persist/backend — the injected storage protocol. Every facet (log, kv,
+; snapshot) goes through a backend dict, never touching storage directly, so
+; file/pg/ipfs-ref backends swap in unchanged. A backend is a dict of fns:
+;   {:append :read :kv-get :kv-put :kv-delete :kv-has? :kv-keys}
+; The in-memory backend is the test default. State is two dicts held in a
+; closure and mutated with set!: logs (stream -> event list) and kv.
+
+(define
+  persist/mem-backend
+  (fn () (let ((logs {}) (kv {})) {:kv-keys (fn () (keys kv)) :read (fn (stream) (let ((cur (get logs stream))) (if cur cur (list)))) :kv-has? (fn (key) (has-key? kv key)) :append (fn (stream event) (let ((cur (get logs stream))) (set! logs (assoc logs stream (append (if cur cur (list)) event))))) :kv-delete (fn (key) (set! kv (dissoc kv key))) :kv-put (fn (key val) (set! kv (assoc kv key val))) :kv-get (fn (key) (get kv key))})))
+
+; protocol accessors — call a backend op by keyword
+(define
+  persist/backend-append
+  (fn (b stream event) ((get b :append) stream event)))
+(define persist/backend-read (fn (b stream) ((get b :read) stream)))
+(define persist/backend-kv-get (fn (b key) ((get b :kv-get) key)))
+(define persist/backend-kv-put (fn (b key val) ((get b :kv-put) key val)))
+(define persist/backend-kv-delete (fn (b key) ((get b :kv-delete) key)))
+(define persist/backend-kv-has? (fn (b key) ((get b :kv-has?) key)))
+(define persist/backend-kv-keys (fn (b) ((get b :kv-keys))))