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>

lib/feed/dedupe.sx

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+; feed/dedupe — collapse duplicate items, keeping first occurrence per key.
+; Each verb may want its own key (see briefing): "alice posted X" keys on
+; (actor verb object) — distinct per actor; "alice liked X / bob liked X"
+; collapse on (verb object) so the cross-actor likes fold into one.
+;
+; Requires: lib/feed/normalize.sx, lib/feed/stream.sx, lib/feed/fanout.sx
+; (feed/-elem? lives in fanout.sx).
+
+; generic: dedupe a stream by key-fn, first occurrence wins (stable)
+(define
+  feed/-dedup-by
+  (fn
+    (items key-fn)
+    (get
+      (reduce
+        (fn
+          (st x)
+          (let
+            ((k (key-fn x)))
+            (if (feed/-elem? k (get st :seen)) st {:seen (append (get st :seen) (list k)) :out (append (get st :out) (list x))})))
+        {:seen (list) :out (list)}
+        items)
+      :out)))
+
+(define
+  feed/dedupe
+  (fn
+    (stream key-fn)
+    (feed/stream (feed/-dedup-by (feed/items stream) key-fn))))
+
+; --- keys -------------------------------------------------------------------
+
+(define
+  feed/activity-key
+  (fn (a) (list (get a :actor) (get a :verb) (get a :object))))
+
+; collapse cross-actor duplicates of the same verb+object (e.g. likes)
+(define feed/collapse-key (fn (a) (list (get a :verb) (get a :object))))
+
+; per-receiver inbox key — one inbox event per (receiver, actor, verb, object)
+(define
+  feed/event-key
+  (fn
+    (ev)
+    (let
+      ((a (get ev :activity)))
+      (list (get ev :to) (get a :actor) (get a :verb) (get a :object)))))
+
+; --- ready-made dedupers ----------------------------------------------------
+
+(define feed/dedupe-activities (fn (s) (feed/dedupe s feed/activity-key)))
+
+(define feed/dedupe-collapse (fn (s) (feed/dedupe s feed/collapse-key)))
+
+; dedupe an inbox: at most one event per receiver per (actor verb object)
+(define feed/dedupe-inbox (fn (inbox) (feed/dedupe inbox feed/event-key)))