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/log.sx

@@ -0,0 +1,37 @@
+; persist/log — the log facet: append-only event streams. seq is assigned
+; sequentially per stream (1-based). Reads return events oldest-first.
+; Requires: lib/persist/event.sx, lib/persist/backend.sx.
+
+; current length of a stream
+(define
+  persist/stream-len
+  (fn (b stream) (len (persist/backend-read b stream))))
+
+; last seq in a stream (0 if empty)
+(define persist/last-seq (fn (b stream) (persist/stream-len b stream)))
+
+; append an event, auto-assigning the next seq. Returns the stored event.
+(define
+  persist/append
+  (fn
+    (b stream type at data)
+    (let
+      ((seq (+ 1 (persist/last-seq b stream))))
+      (let
+        ((ev (persist/event stream seq type at data)))
+        (begin (persist/backend-append b stream ev) ev)))))
+
+; read all events in a stream, oldest-first
+(define persist/read (fn (b stream) (persist/backend-read b stream)))
+
+; read events with seq >= from
+(define
+  persist/read-from
+  (fn
+    (b stream from)
+    (filter
+      (fn (e) (>= (persist/event-seq e) from))
+      (persist/read b stream))))
+
+; number of events in a stream
+(define persist/count (fn (b stream) (persist/stream-len b stream)))