feed: TF-IDF content ranking over :tags — tag-df/idf, tfidf-score, by-relevance + 15 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` → **105/105** (Phases 1–4 complete)
+`bash lib/feed/conformance.sh` → **120/120** (Phases 1–4 + TF-IDF complete)
 
 ## Ground rules
 
@@ -138,7 +138,9 @@ are function parameters. Real acl-sx / fed-sx wire in at the call site unchanged
 
 - Wire real acl-sx once `lib/acl/` exists (swap injected `permit?`).
 - Wire real fed-sx transport (swap `send-fn`/`fetch-fn`).
-- TF-IDF over `:tags` for content ranking (sketch mentions it; not yet built).
+- [x] TF-IDF over `:tags` for content ranking — `content.sx`: `feed/tag-df`,
+  `feed/tag-idf` (log N/df), `feed/tfidf-score`, `feed/by-relevance`; 15 tests.
+  Composes as a scorer with rank.sx. (120/120 total.)
 - Notification feed (verb-filtered, per-recipient) as a thin layer over fanout.
 
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