persist: global commit ordering across streams + 11 tests

global.sx: persist/gappend records a pointer in a reserved $global index whose
seq is the global commit position; read-global/project-global replay every
event in commit order; global-from for incremental consumers. Opt-in (plain
append untouched); $-prefixed streams now reserved + hidden from the public
catalog (streams-all reveals them). Gives feed its unified timeline.
Deterministic across restart. 191/191.

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

lib/persist/catalog.sx

@@ -1,15 +1,30 @@
 ; 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.
+; whose log has been fully compacted still appears. $-prefixed streams are
+; reserved for internal indexes (e.g. the $global commit index) and are hidden
+; from the public catalog; use streams-all to see them. For admin, global ops,
+; and cross-stream tooling. Requires: lib/persist/backend.sx, lib/persist/log.sx.
+
+(define persist/reserved-stream? (fn (s) (starts-with? s "$")))
+
+; every stream including reserved internal indexes
+(define persist/streams-all (fn (b) (persist/backend-streams b)))
+
+; public streams (reserved internal indexes hidden)
+(define
+  persist/streams
+  (fn
+    (b)
+    (filter
+      (fn (s) (not (persist/reserved-stream? s)))
+      (persist/streams-all b))))
 
-(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)
+; total logical events across all public streams (sum of high-water marks)
 (define
   persist/total-events
   (fn