persist: stream catalog — enumerate streams + 10 tests

New backend op :streams (from seq high-water marks, so compacted streams still
list), threaded through mem-backend + durable serve/io-backend. catalog.sx:
persist/streams, stream-count, stream-exists?, total-events. 143/143.

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

lib/persist/scoreboard.json

@@ -12,9 +12,10 @@
     "blob": {"pass": 14, "fail": 0},
     "view": {"pass": 11, "fail": 0},
     "cas": {"pass": 11, "fail": 0},
+    "catalog": {"pass": 10, "fail": 0},
     "recovery": {"pass": 6, "fail": 0}
   },
-  "total_pass": 133,
+  "total_pass": 143,
   "total_fail": 0,
-  "total": 133
+  "total": 143
 }