feed: pagination — offset/limit + cursor-by-at (before/after/page-before/next-cursor) + 14 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` → **163/163** (Phases 1–4 + TF-IDF, notifications, home, smart-dedupe, trending, mute)
+`bash lib/feed/conformance.sh` → **177/177** (Phases 1–4 + TF-IDF, notifications, home, smart-dedupe, trending, mute, pagination)
 
 ## Ground rules
 
@@ -157,6 +157,9 @@ are function parameters. Real acl-sx / fed-sx wire in at the call site unchanged
 - [x] Mute/block — `feed/mute-actors` / `feed/mute-tags` / `feed/mute-objects` /
   `feed/apply-prefs`: viewer-controlled per-request filtering (complements ACL's
   author-controlled visibility) (`mute.sx`); 9 tests. (163/163 total.)
+- [x] Pagination — `feed/page`/`feed/page-count` (offset) + `feed/before`/
+  `feed/after`/`feed/page-before`/`feed/next-cursor` (cursor by :at, stable under
+  inserts) (`page.sx`); 14 tests. (177/177 total.)
 
 (none)