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/catalog.sx

@@ -0,0 +1,20 @@
+; persist/catalog — enumerate the streams a backend holds. The catalog is the
+; set of streams ever appended to (from the seq high-water marks), so a stream
+; whose log has been fully compacted still appears. For admin, global ops, and
+; cross-stream tooling. Requires: lib/persist/backend.sx, lib/persist/log.sx.
+
+(define persist/streams (fn (b) (persist/backend-streams b)))
+(define persist/stream-count (fn (b) (len (persist/streams b))))
+(define
+  persist/stream-exists?
+  (fn (b stream) (contains? (persist/streams b) stream)))
+
+; total logical events across all streams (sum of high-water marks)
+(define
+  persist/total-events
+  (fn
+    (b)
+    (reduce
+      (fn (acc s) (+ acc (persist/last-seq b s)))
+      0
+      (persist/streams b))))