persist: projections — fold stream into read model, incremental resume + 9 tests

project.sx: projection state {:value :seq}; persist/project folds the whole
stream, persist/project-resume folds only the tail so read models update
incrementally. Pure step (value event)->value. 37/37.

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

plans/persist-on-sx.md

@@ -94,7 +94,7 @@ lib/persist/backend.sx              lib/persist/api.sx
 - [x] `api.sx` + tests + scoreboard + conformance.sh
 
 ## Phase 2 — Projections + subscriptions
-- [ ] `project.sx` — `(project stream step seed)`, incremental fold
+- [x] `project.sx` — `(project stream step seed)`, incremental fold
 - [ ] subscription hook — projection / kv read model re-runs on append
 - [ ] concurrency conflict surfaced as a real result, not a crash
 
@@ -113,6 +113,10 @@ feed/-log, flow store, mod/audit, search index, acl grants, identity sessions al
 become `persist` log or kv. Track each migration in that subsystem's plan.
 
 ## Progress log
+- **Phase 2a (37/37).** `project.sx` — projection state `{:value :seq}`;
+  `persist/project` folds whole stream from seed, `persist/project-resume`
+  folds only the tail (seq > prior seq) so read models update incrementally.
+  step is pure `(value event) -> value`. 9 tests incl. resume==full-from-zero.
 - **Phase 1 complete (28/28).** `event.sx` (event record + accessors),
   `backend.sx` (injectable protocol + in-memory log/kv impl, closure state via
   set!), `log.sx` (append/read/read-from, sequential per-stream seq, stream