The dev mock's Identity/set discarded its payload and Identity/get always
returned a static list, so saved identities never round-tripped in local
development. Persist create (with mayDelete: true), update, and destroy in
place, mirroring handleMailboxSet, so signature edits stick when testing
without a real JMAP server.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>