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>

plans/persist-on-sx.md

@@ -42,7 +42,7 @@ read models (feeds, indices, audit logs) update incrementally.
 
 ## Status (rolling)
 
-`bash lib/persist/conformance.sh` → **180/180** (Phases 1–4 complete + extensions)
+`bash lib/persist/conformance.sh` → **191/191** (Phases 1–4 complete + extensions)
 
 ## Ground rules
 
@@ -177,11 +177,25 @@ over an in-process disk (the mock-IO harness).
   keyed by a caller idempotency key (per stream), returning the same event on a
   repeat. Marker lives in kv, so idempotency holds across restart. `seen?` check.
 
+- [x] `global.sx` — global commit ordering across streams (the primitive feed's
+  unified timeline needs). `persist/gappend` records a pointer in a reserved
+  `$global` index whose seq is the commit position; `read-global`/
+  `project-global` replay every event in commit order; `global-from` for
+  incremental consumers. Opt-in (plain `append` never touches it); reserved
+  index hidden from the public catalog. Deterministic across restart.
+
 ## Consumers (post-foundation, not in scope here)
 feed/-log, flow store, mod/audit, search index, acl grants, identity sessions all
 become `persist` log or kv. Track each migration in that subsystem's plan.
 
 ## Progress log
+- **Ext: global commit ordering (191/191).** `global.sx` — `persist/gappend`
+  records a pointer in a reserved `$global` index (its seq = global commit
+  position); `read-global`/`project-global` resolve pointers to events in commit
+  order; `global-from` for incremental global consumers. Opt-in; `$`-streams are
+  now reserved + hidden from the public catalog (`streams-all` reveals them).
+  Gives feed its cross-stream timeline. 11 tests incl. durable + restart
+  determinism.
 - **Ext: exactly-once append (180/180).** `idempotency.sx` —
   `persist/append-once` appends at most once per (stream, idempotency key),
   returning the same event on a repeat; the marker lives in kv so it survives