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/conformance.sh

@@ -13,7 +13,7 @@ if [ ! -x "$SX_SERVER" ]; then
   exit 1
 fi
 
-SUITES=(event log kv project)
+SUITES=(event log kv project subscribe)
 
 OUT_JSON="lib/persist/scoreboard.json"
 OUT_MD="lib/persist/scoreboard.md"
@@ -32,6 +32,7 @@ run_suite() {
 (load "lib/persist/log.sx")
 (load "lib/persist/kv.sx")
 (load "lib/persist/project.sx")
+(load "lib/persist/subscribe.sx")
 (load "lib/persist/api.sx")
 (epoch 2)
 (eval "(define persist-test-pass 0)")