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/tests/thread.sx

@@ -0,0 +1,49 @@
+; Follow-up — conversation threading via :reply-to closure. (feed-test name got expected)
+
+(define
+  S
+  (feed/stream
+    (list
+      (feed/normalize {:actor "a" :object "root" :at 1})
+      (feed/normalize {:actor "b" :object "r1" :at 2 :verb "reply" :reply-to "root"})
+      (feed/normalize {:actor "c" :object "r2" :at 3 :verb "reply" :reply-to "root"})
+      (feed/normalize {:actor "d" :object "r3" :at 4 :verb "reply" :reply-to "r1"})
+      (feed/normalize {:actor "e" :object "x" :at 5}))))
+
+; ---------- direct replies ----------
+
+(feed-test "direct replies to root" (feed/reply-count S "root") 2)
+(feed-test "direct replies to r1" (feed/reply-count S "r1") 1)
+(feed-test "no replies to r3" (feed/reply-count S "r3") 0)
+(feed-test
+  "replies objects to root"
+  (map (fn (a) (get a :object)) (feed/items (feed/replies S "root")))
+  (list "r1" "r2"))
+
+; ---------- thread closure ----------
+
+(feed-test
+  "thread objects root (transitive)"
+  (feed/thread-objects S "root")
+  (list "root" "r1" "r2" "r3"))
+(feed-test
+  "thread root chronological"
+  (map (fn (a) (get a :object)) (feed/items (feed/thread S "root")))
+  (list "root" "r1" "r2" "r3"))
+(feed-test "thread size root" (feed/thread-size S "root") 4)
+(feed-test
+  "thread excludes unrelated x"
+  (feed/-elem?
+    "x"
+    (map (fn (a) (get a :object)) (feed/items (feed/thread S "root"))))
+  false)
+
+; ---------- sub-thread ----------
+
+(feed-test
+  "thread from r1 (sub-tree)"
+  (map (fn (a) (get a :object)) (feed/items (feed/thread S "r1")))
+  (list "r1" "r3"))
+(feed-test "thread size r1" (feed/thread-size S "r1") 2)
+(feed-test "leaf thread is itself" (feed/thread-size S "r3") 1)
+(feed-test "unrelated thread is itself" (feed/thread-size S "x") 1)