The E2EE direct-message types (MessageUser/Message/Conversation/MyKeyBundle/
KeySetup) + the ~9 messaging methods move into a self-contained
api/messaging.ts exporting a `messagingApi` slice (imports only core's req).
api.ts spreads the slice and re-exports the types. api.ts 882 -> 819 lines; no
runtime change. Gate green: typecheck, eslint, prettier, 74 vitest.