radar: pass 3 — offset/limit pagination (feed+search, 2 consumers, 1-liner) folded into W3 → substrate

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

plans/abstractions.md

@@ -12,7 +12,11 @@ Anything short → _Watching_ (what's missing) or _Rejected_ (why).
 
 ## Last scan
 
-- **Date:** 2026-06-06 (radar loop, pass 2)
+- **Date:** 2026-06-06 (radar loop, pass 3)
+- **Pass 3:** subsystem set + tmux + A1 adopters (4) all unchanged vs pass 2. Loops
+  advanced: acl shipped Phase 4 federation; search shipped Phase 4 + pagination; feed
+  shipped pagination/threading; mod at Ext 19 (capstone); persist did a worked acl-grants
+  migration (W4). New shape found: offset/limit pagination → folded into W3.
 - **Subsystem set discovered:** loop worktrees `acl, erlang, fed-prims, fed-sx-m1,
   feed, flow, go, kernel, mod, ocaml, persist, radar, ruby, search,
   sx-vm-extensions`; main-repo `lib/*` incl. merged `feed` + substrates (`apl,
@@ -103,11 +107,21 @@ Anything short → _Watching_ (what's missing) or _Rejected_ (why).
 - **Home when ripe:** delegate `permit?` to `acl-on-sx`; do NOT extract the filter.
   Re-check if a 3rd genuine per-viewer read filter ships (e.g. events/commerce).
 
-### W3 · Collection helpers (group-by, dedupe-by-key, stable top-N, distinct-order)
+### W3 · Collection helpers (group-by, dedupe-by-key, stable top-N, distinct-order, offset/limit page)
 - feed built all of these on APL primitives. search/commerce/events will want
   group-by / top-N.
-- **Missing:** ≥3 stable consumers; and these likely belong in the **substrate**
-  (APL/Haskell already expose grade/sort/unique) rather than a shared lib. Watch.
+- **NEW (2026-06-06): offset/limit pagination shipped in 2 subsystems, identical shape**
+  `take limit (drop offset xs)`:
+  - `feed/lib/feed/page.sx:9` `feed/page` (offset/limit window over a stream).
+  - `search/lib/search/page.sx:9` `paginate off lim docs = take lim (drop off docs)`.
+  - NOT a 3rd: `persist/lib/persist/query.sx:5` has a *since-cursor* for incremental log
+    consumption — resumable-stream semantics, not result windowing. Different shape.
+  - feed *also* has cursor-by-`:at` recency pagination (`page.sx:21-44`); search has no
+    cursor. So only the plain offset/limit window is shared, and it is a literal 1-liner.
+- **Missing:** ≥3 stable consumers; AND every item here is collection math that belongs
+  in the **substrate** (APL/Haskell already expose grade/sort/unique/take/drop), not a
+  shared lib. A 1-line `take/drop` window is far below the extraction threshold. Watch;
+  revisit only if a non-substrate subsystem needs the same windowing without take/drop.
 
 ### W4 · In-memory store fakes → `persist-on-sx`
 - Not an abstraction to extract — a migration target. Every subsystem fakes its