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>

plans/feed-on-sx.md

@@ -14,7 +14,7 @@ APL, ACL visibility filtering via `lib/acl/`, federation via fed-sx.
 
 ## Status (rolling)
 
-`bash lib/feed/conformance.sh` → **120/120** (Phases 1–4 + TF-IDF complete)
+`bash lib/feed/conformance.sh` → **128/128** (Phases 1–4 + TF-IDF + notifications)
 
 ## Ground rules
 
@@ -141,7 +141,9 @@ are function parameters. Real acl-sx / fed-sx wire in at the call site unchanged
 - [x] TF-IDF over `:tags` for content ranking — `content.sx`: `feed/tag-df`,
   `feed/tag-idf` (log N/df), `feed/tfidf-score`, `feed/by-relevance`; 15 tests.
   Composes as a scorer with rank.sx. (120/120 total.)
-- Notification feed (verb-filtered, per-recipient) as a thin layer over fanout.
+- [x] Notification feed (verb-filtered, per-recipient) — `notify.sx`:
+  `feed/notifications`, `feed/notify-verbs`, `feed/notify-digest` (collapses
+  "X, Y liked Z" by (verb,object), sorted-deterministic); 8 tests. (128/128 total.)
 
 (none)