Break KVStorageComposite into focused classes

- KVReplicaSet: seed, apply, local scan, diff against the source, sync meta
- KVOutbox: persisted offline queue with in-order replay (load() resumes it)
- utils: KeyRange (also used for server authorization), KeyCounter,
  SerialQueue (also used by KVStorageHttp and the event-sourced store),
  RetryTimer (separate timers for sync and replay; previously one shared
  timer could cancel the other's retry)

The composite now only orchestrates. Behaviour unchanged; new unit tests for
the building blocks and the outbox.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017Qxbkp4tA7G5SCWjzWuxmE

src/index.ts

@@ -4,6 +4,8 @@ export * from './kv-storage/kv-storage-http.js'
 export * from './kv-storage/kv-storage-http-server.js'
 export * from './kv-storage/kv-storage-indexeddb.js'
 export * from './kv-storage/kv-storage-composite.js'
+export * from './kv-storage/kv-replica-set.js'
+export * from './kv-storage/kv-outbox.js'
 export * from './event-storage/base-event-storage.js'
 export * from './event-storage/event-storage-kv.js'
 export * from './document-storage/base-document-storage.js'
@@ -17,6 +19,10 @@ export * from './utils/errors.js'
 export * from './utils/event-emitter.js'
 export * from './utils/extended-json.js'
 export * from './utils/hex.js'
+export * from './utils/key-counter.js'
+export * from './utils/key-range.js'
+export * from './utils/retry-timer.js'
+export * from './utils/serial-queue.js'
 export * from './utils/request-auth.js'
 export * from './utils/json.js'
 export * from './utils/keys.js'