persist: subscription hub — read models update on publish + 9 tests

subscribe.sx: persist/hub wraps a backend; persist/publish appends then fires
per-stream callbacks (backend stream event). Direct persist/append bypasses
subscribers (bulk load/replay). Callbacks drive kv counters / project-resume. 46/46.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/persist/tests/subscribe.sx

@@ -0,0 +1,130 @@
+; Phase 2 — subscription hub: callbacks fire on publish, drive read models.
+
+(persist-test
+  "no subscribers initially"
+  (persist/subscriber-count (persist/hub (persist/open)) "s")
+  0)
+(persist-test
+  "subscribe registers a callback"
+  (let
+    ((h (persist/hub (persist/open))))
+    (begin
+      (persist/subscribe h "s" (fn (b s e) nil))
+      (persist/subscriber-count h "s")))
+  1)
+(persist-test
+  "publish appends to the log"
+  (let
+    ((b (persist/open)))
+    (let
+      ((h (persist/hub b)))
+      (begin
+        (persist/publish h "s" "x" 0 {})
+        (persist/publish h "s" "x" 0 {})
+        (persist/count b "s"))))
+  2)
+(persist-test
+  "publish returns the stored event"
+  (let
+    ((h (persist/hub (persist/open))))
+    (persist/event-seq (persist/publish h "s" "x" 0 {:id 1})))
+  1)
+(persist-test
+  "callback fires on publish — drives a kv read model"
+  (let
+    ((b (persist/open)))
+    (let
+      ((h (persist/hub b)))
+      (begin
+        (persist/subscribe
+          h
+          "s"
+          (fn
+            (bk s e)
+            (persist/kv-update
+              bk
+              "count"
+              0
+              (fn (n) (+ n 1)))))
+        (persist/publish h "s" "x" 0 {})
+        (persist/publish h "s" "x" 0 {})
+        (persist/publish h "s" "x" 0 {})
+        (persist/kv-get b "count"))))
+  3)
+(persist-test
+  "callback receives the event"
+  (let
+    ((b (persist/open)))
+    (let
+      ((h (persist/hub b)))
+      (begin
+        (persist/subscribe
+          h
+          "s"
+          (fn (bk s e) (persist/kv-put bk "last" (persist/event-type e))))
+        (persist/publish h "s" "created" 0 {})
+        (persist/kv-get b "last"))))
+  "created")
+(persist-test
+  "subscriptions are per-stream"
+  (let
+    ((b (persist/open)))
+    (let
+      ((h (persist/hub b)))
+      (begin
+        (persist/subscribe
+          h
+          "s1"
+          (fn
+            (bk s e)
+            (persist/kv-update bk "n" 0 (fn (n) (+ n 1)))))
+        (persist/publish h "s2" "x" 0 {})
+        (persist/kv-get-or b "n" 0))))
+  0)
+(persist-test
+  "multiple subscribers all fire"
+  (let
+    ((b (persist/open)))
+    (let
+      ((h (persist/hub b)))
+      (begin
+        (persist/subscribe
+          h
+          "s"
+          (fn
+            (bk s e)
+            (persist/kv-update bk "a" 0 (fn (n) (+ n 1)))))
+        (persist/subscribe
+          h
+          "s"
+          (fn
+            (bk s e)
+            (persist/kv-update bk "b" 0 (fn (n) (+ n 10)))))
+        (persist/publish h "s" "x" 0 {})
+        (list (persist/kv-get b "a") (persist/kv-get b "b")))))
+  (list 1 10))
+(persist-test
+  "incremental read model via resume in callback"
+  (let
+    ((b (persist/open)))
+    (let
+      ((h (persist/hub b)))
+      (begin
+        (persist/kv-put b "proj" {:value 0 :seq 0})
+        (persist/subscribe
+          h
+          "s"
+          (fn
+            (bk s e)
+            (persist/kv-put
+              bk
+              "proj"
+              (persist/project-resume
+                bk
+                s
+                (fn (acc ev) (+ acc 1))
+                (persist/kv-get bk "proj")))))
+        (persist/publish h "s" "x" 0 {})
+        (persist/publish h "s" "x" 0 {})
+        (persist/project-value (persist/kv-get b "proj")))))
+  2)