feed: notification feed — per-recipient inbox, verb filter, (verb,object) digest + 8 tests

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

lib/feed/notify.sx

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+; feed/notify — a notification feed is a thin layer over a recipient's inbox:
+; the events directed at a user, optionally verb-filtered, and a digest that
+; collapses "alice, bob and 1 other liked X" by (verb, object).
+;
+; Requires: lib/feed/normalize.sx, lib/feed/stream.sx, lib/feed/fanout.sx
+; (feed/inbox-for, feed/-elem?).
+
+; all inbox events for a user (their raw notifications)
+(define feed/notifications (fn (inbox user) (feed/inbox-for inbox user)))
+
+; restrict to notification-worthy verbs (e.g. (list "like" "reply" "follow"))
+(define
+  feed/notify-verbs
+  (fn
+    (inbox user verbs)
+    (feed/filter
+      (feed/inbox-for inbox user)
+      (fn (ev) (feed/-elem? (get (get ev :activity) :verb) verbs)))))
+
+; group key "verb|object" — deterministic, sortable
+(define
+  feed/-notify-key
+  (fn
+    (ev)
+    (let
+      ((a (get ev :activity)))
+      (string-append (get a :verb) "|" (get a :object)))))
+
+; digest: one entry per (verb, object) with the distinct actors and a count,
+; ordered by key for determinism.
+(define
+  feed/notify-digest
+  (fn
+    (inbox user)
+    (let
+      ((events (feed/items (feed/inbox-for inbox user))))
+      (let
+        ((groups (reduce (fn (g ev) (let ((a (get ev :activity)) (k (feed/-notify-key ev))) (let ((cur (get g k {:object (get a :object) :actors (list) :verb (get a :verb)}))) (assoc g k (assoc cur :actors (append (get cur :actors) (list (get a :actor)))))))) {} events)))
+        (map
+          (fn
+            (k)
+            (let
+              ((grp (get groups k)))
+              (assoc grp :count (len (get grp :actors)))))
+          (sort (keys groups)))))))