feed: conversation threading — :reply-to transitive closure (thread/replies/thread-size) + 12 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 rank integration content notify home dedupe trending mute page)
+SUITES=(basic fanout rank integration content notify home dedupe trending mute page thread)
 
 OUT_JSON="lib/feed/scoreboard.json"
 OUT_MD="lib/feed/scoreboard.md"
@@ -43,6 +43,7 @@ run_suite() {
 (load "lib/feed/trending.sx")
 (load "lib/feed/mute.sx")
 (load "lib/feed/page.sx")
+(load "lib/feed/thread.sx")
 (epoch 2)
 (eval "(define feed-test-pass 0)")
 (eval "(define feed-test-fail 0)")