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/document-storage/event-sourced-document-storage.ts

@@ -3,6 +3,7 @@ import type { Doc } from '../utils/document-path.js'
 import type { DocumentQuery, FindResult } from '../utils/document-query.js'
 import { DocumentUpdate, type DocChange } from '../utils/document-update.js'
 import { ConcurrencyError } from '../utils/errors.js'
+import { SerialQueue } from '../utils/serial-queue.js'
 import type { AnySpec, ChildOf, KeyOf, Spec, ValueAt } from '../utils/schema.js'
 import { BaseDocumentStorage } from './base-document-storage.js'
 
@@ -29,8 +30,8 @@ export interface EventSourcedDocumentStorage<S extends Spec = AnySpec> {
  */
 export class EventSourcedDocumentStorage<S extends Spec = AnySpec> extends BaseDocumentStorage<S> {
   private readonly maxRetries: number
-  /** Tail of the pending writes per document, so writes in one process don't conflict with each other. */
-  private readonly locks = new Map<string, Promise<unknown>>()
+  /** Writes to one document run one at a time, so writes in one process don't conflict with each other. */
+  private readonly writes = new SerialQueue()
 
   constructor(
     private readonly events: BaseEventStorage,
@@ -50,12 +51,7 @@ export class EventSourcedDocumentStorage<S extends Spec = AnySpec> extends BaseD
   }
 
   mutate(path: readonly string[], change: (current: Doc | undefined) => DocChange | null): Promise<Doc | undefined> {
-    const id = JSON.stringify(path)
-    const result = (this.locks.get(id) ?? Promise.resolve()).then(() => this.write(path, change))
-    const tail = result.catch(() => {})
-    this.locks.set(id, tail)
-    void tail.then(() => this.locks.get(id) === tail && this.locks.delete(id))
-    return result
+    return this.writes.run(() => this.write(path, change), JSON.stringify(path))
   }
 
   private async write(path: readonly string[], change: (current: Doc | undefined) => DocChange | null): Promise<Doc | undefined> {