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>

lib/feed/conformance.sh

@@ -13,7 +13,7 @@ if [ ! -x "$SX_SERVER" ]; then
   exit 1
 fi
 
-SUITES=(basic fanout)
+SUITES=(basic fanout rank)
 
 OUT_JSON="lib/feed/scoreboard.json"
 OUT_MD="lib/feed/scoreboard.md"
@@ -33,6 +33,8 @@ run_suite() {
 (load "lib/feed/api.sx")
 (load "lib/feed/fanout.sx")
 (load "lib/feed/dedupe.sx")
+(load "lib/feed/aggregate.sx")
+(load "lib/feed/rank.sx")
 (epoch 2)
 (eval "(define feed-test-pass 0)")
 (eval "(define feed-test-fail 0)")