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/event.sx

@@ -0,0 +1,13 @@
+; persist/event — an event is the unit of the log facet:
+;   {:stream :seq :type :at :data}
+; stream = which append-only stream, seq = 1-based position within it,
+; type = event kind, at = caller-supplied timestamp (never a clock here:
+; replay must stay pure), data = payload dict.
+
+(define persist/event (fn (stream seq type at data) {:data data :type type :at at :stream stream :seq seq}))
+
+(define persist/event-stream (fn (e) (get e :stream)))
+(define persist/event-seq (fn (e) (get e :seq)))
+(define persist/event-type (fn (e) (get e :type)))
+(define persist/event-at (fn (e) (get e :at)))
+(define persist/event-data (fn (e) (get e :data)))