feed: Phase 3 aggregation + ranking — group-by, recency/velocity/engagement scorers, composite, top-N via stable grade-down + 24 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` → **59/59** (Phases 1–2 complete)
+`bash lib/feed/conformance.sh` → **83/83** (Phases 1–3 complete)
 
 ## Ground rules
 
@@ -81,13 +81,14 @@ lib/feed/api.sx           lib/feed/fed.sx
 
 ## Phase 3 — Aggregation + ranking
 
-- [ ] group-by — `(actor, day) → count` via key-reduce
-- [ ] velocity score — recent activity count over window
-- [ ] recency score — decay by age
-- [ ] composite rank — weighted sum of components
-- [ ] top-N per timeline
-- [ ] `lib/feed/tests/rank.sx` — 20+ cases: ranking stable on tie, decay shape,
-  per-user weighting
+- [x] group-by — `feed/group-by`/`feed/group-count` key-reduce; `feed/by-actor-day`
+  buckets `(actor, day)` via `feed/day` (string-joined keys)
+- [x] velocity score — `feed/velocity` counts actor's activities in `(at-window, at]`
+- [x] recency score — `feed/recency` half-life decay `0.5^(age/hl)`
+- [x] composite rank — `feed/composite` weighted sum of `(weight scorer)` parts
+- [x] top-N per timeline — `feed/top` = rank then take
+- [x] `lib/feed/tests/rank.sx` — 24 cases: decay shape, velocity burst, stable
+  tie-break, top-N, composite
 
 ## Phase 4 — Visibility filter + federation
 
@@ -114,6 +115,11 @@ lib/feed/api.sx           lib/feed/fed.sx
   `feed/audience` sorts (else recipient ordering flakes). `apl-compress` needs a
   rank-1 array, so the (activity×follower) matrix is flattened to its ravel before
   the edge-guard filter.
+- **Phase 3 done (83/83 total).** `aggregate.sx` (group-by/count, day buckets) +
+  `rank.sx` (recency/velocity/engagement scorers, composite, top-N). `sort` is
+  single-arg ascending only — no comparator — so ranking uses a stable two-pass
+  `apl-grade-down` (by :at desc, then by score desc) for deterministic tie-breaks.
+  Dict keys must be strings, so composite group keys are string-joined ("actor#day").
 
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