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/scoreboard.json

@@ -17,9 +17,10 @@
     "batch": {"pass": 10, "fail": 0},
     "upcast": {"pass": 9, "fail": 0},
     "idempotency": {"pass": 9, "fail": 0},
+    "global": {"pass": 11, "fail": 0},
     "recovery": {"pass": 6, "fail": 0}
   },
-  "total_pass": 180,
+  "total_pass": 191,
   "total_fail": 0,
-  "total": 180
+  "total": 191
 }