feed: Phase 2 fanout via outer product — activities ∘.× audience, flatten, edge-guard, dedupe + 29 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` → **30/30** (Phase 1 complete)
+`bash lib/feed/conformance.sh` → **59/59** (Phases 1–2 complete)
 
 ## Ground rules
 
@@ -69,13 +69,15 @@ lib/feed/api.sx           lib/feed/fed.sx
 
 ## Phase 2 — Fanout via outer product
 
-- [ ] follower graph: `followers user → vector of user ids`
-- [ ] fanout: activities `∘.×` followers → matrix `(activity, follower)` pairs
-- [ ] flatten to inbox events vector
-- [ ] dedupe — group by `(actor, verb, object)` collapse to one inbox event per
-  receiver
-- [ ] `lib/feed/tests/fanout.sx` — 20+ cases: small graph, mutual follow, popular
-  actor (high-fanout), cross-post dedupe
+- [x] follower graph: `followers user → vector of user ids` (`feed/follow-graph`,
+  `feed/followers`; graph = `{followee -> (followers)}` dict)
+- [x] fanout: activities `∘.×` audience → matrix via `apl-outer feed/-mk-event`
+- [x] flatten to inbox events vector (`feed/-flatten` rank-2 → rank-1)
+- [x] dedupe — `feed/dedupe-inbox` by `(to, actor, verb, object)`; also
+  `feed/dedupe-activities` `(actor verb object)` and `feed/dedupe-collapse`
+  `(verb object)` for cross-actor likes
+- [x] `lib/feed/tests/fanout.sx` — 29 cases: small graph, mutual follow, star
+  (high-fanout), empty graph, unfollowed actor, cross-post dedupe
 
 ## Phase 3 — Aggregation + ranking
 
@@ -104,8 +106,14 @@ lib/feed/api.sx           lib/feed/fed.sx
   by-actor/verb/object/since predicates), `api.sx` (mutable log: post/all/reset!/size).
   Substrate: `apl-compress`, `apl-grade-up`, `apl-take`, `apl-reverse`, `make-array`.
   Grade-up returns 1-based indices (⎕IO=1), is stable on ties → deterministic sort.
-
-## Blockers
+- **Phase 2 done (59/59 total).** `fanout.sx` (graph + `apl-outer` showcase),
+  `dedupe.sx` (per-key dedupe, first-wins stable). Key APL gotcha: `scalar?` is
+  true for ANY dict and `disclose` nils a non-array dict, so an apl-outer combiner
+  MUST `enclose` its event dict — apl-outer discloses it back intact. `apl-unique`
+  preserves first-occurrence order; dict `keys` order is NOT stable, so
+  `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.
 
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