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/subscribe.sx

@@ -0,0 +1,21 @@
+; persist/subscribe — a subscription hub wraps a backend with per-stream
+; callbacks fired after each append. The canonical use: a callback re-runs a
+; projection (or bumps a kv counter) so read models update incrementally on
+; write instead of being recomputed on read.
+;   callback signature: (backend stream event) -> ignored
+; Publish goes through the hub; direct persist/append on the backend bypasses
+; subscribers by design (bulk loads, replay).
+; Requires: lib/persist/log.sx.
+
+(define persist/hub (fn (b) (let ((subs {})) {:subscriber-count (fn (stream) (let ((cs (get subs stream))) (if cs (len cs) 0))) :publish (fn (stream type at data) (let ((ev (persist/append b stream type at data))) (begin (for-each (fn (cb) (cb b stream ev)) (let ((cs (get subs stream))) (if cs cs (list)))) ev))) :subscribe (fn (stream cb) (let ((cur (get subs stream))) (set! subs (assoc subs stream (append (if cur cur (list)) cb))))) :backend b})))
+
+(define persist/hub-backend (fn (h) (get h :backend)))
+(define
+  persist/subscribe
+  (fn (h stream cb) ((get h :subscribe) stream cb)))
+(define
+  persist/publish
+  (fn (h stream type at data) ((get h :publish) stream type at data)))
+(define
+  persist/subscriber-count
+  (fn (h stream) ((get h :subscriber-count) stream)))