persist: projections — fold stream into read model, incremental resume + 9 tests

project.sx: projection state {:value :seq}; persist/project folds the whole
stream, persist/project-resume folds only the tail so read models update
incrementally. Pure step (value event)->value. 37/37.

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

lib/persist/project.sx

@@ -0,0 +1,30 @@
+; persist/project — a projection folds a stream's events into a read model.
+; A projection state is {:value v :seq s} where s is the last seq folded in,
+; so a projection can resume incrementally from where it left off (replay only
+; the tail). step : (value event) -> value. Determinism: step must be pure —
+; time lives on the event (event-at), never a clock here.
+; Requires: lib/persist/event.sx, lib/persist/log.sx.
+
+; fold the tail (events with seq > prior's seq) onto a prior projection state
+(define
+  persist/project-resume
+  (fn
+    (b stream step prior)
+    (let
+      ((tail (persist/read-from b stream (+ 1 (get prior :seq)))))
+      (reduce (fn (acc e) {:value (step (get acc :value) e) :seq (persist/event-seq e)}) prior tail))))
+
+; project the whole stream from seed
+(define
+  persist/project
+  (fn (b stream step seed) (persist/project-resume b stream step {:value seed :seq 0})))
+
+(define persist/project-value (fn (p) (get p :value)))
+(define persist/project-seq (fn (p) (get p :seq)))
+
+; convenience: project and return just the value
+(define
+  persist/project-fold
+  (fn
+    (b stream step seed)
+    (persist/project-value (persist/project b stream step seed))))