ctx.storage.domain opens declared domains: zod value schemas parsed at
the durable boundary, one write chain per domain (update(fn) is the only
read-modify-write), domain/changed emitted per record after durability
(new snapshot + operation, no old value, per repo event convention).
Domain-to-backend routing is configuration (default backend + per-domain
overrides); unknown names and missing facets fail loud. Ships the
MemoryStorageBackend test helper and a runtime invariant asserting every
change event matches the in-memory state.