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

DESIGN.md

@@ -177,6 +177,18 @@ outbox `~q/o/<seq:12>`.
 - `start()`, `synced()` (complete copy, all received changes applied), `queued` (outbox size),
   `close()`.
 
+**Structure.** The composite only orchestrates (what to read from, when to sync, whether to queue a
+write). The work is done by small classes:
+
+| Class                 | Job                                                                                                                                                  |
+| --------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------- |
+| `KVReplicaSet`        | the replicas of one `KeyRange`: seed the faster ones, apply changes, scan locally (hiding `~q/` bookkeeping), diff against the source, sync metadata |
+| `KVOutbox`            | the persisted offline queue (`~q/o/<seq>`), its pending-key counts, and in-order replay with backoff                                                 |
+| `KeyRange` (utils)    | a key plus its subtree: `contains`, `clamp` scan bounds, `covers` a scan; also used by the server's authorization                                    |
+| `KeyCounter` (utils)  | per-key counts: pending outbox writes and expected echoes                                                                                            |
+| `SerialQueue` (utils) | one-at-a-time tasks, optionally per key: applying remote changes, subscription updates, event-sourced writes                                         |
+| `RetryTimer` (utils)  | backoff retries, one timer per job (full sync and outbox replay retry independently)                                                                 |
+
 ## 5. Event storage
 
 `BaseEventStorage` primitives over stream paths: `appendTo`, `readFrom`, `versionOf`, `streamsOf`.