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

test/kv-storage/kv-outbox.test.ts

@@ -0,0 +1,36 @@
+import { describe, expect, it, vi } from 'vitest'
+import { KVOutbox, type OutboxEntry } from '../../src/kv-storage/kv-outbox.js'
+import { KVStorageInMemory } from '../../src/kv-storage/kv-storage-in-memory.js'
+import { RetryTimer } from '../../src/utils/retry-timer.js'
+import { StorageUnavailableError } from '../../src/utils/errors.js'
+
+describe('KVOutbox', () => {
+  it('replays entries in order once the source is reachable, and survives a restart', async () => {
+    const store = new KVStorageInMemory()
+    let online = false
+    const sent: OutboxEntry[] = []
+    const send = async (entry: OutboxEntry) => {
+      if (!online) throw new StorageUnavailableError('offline')
+      sent.push(entry)
+    }
+    const firstRetry = new RetryTimer({ baseDelay: 5, maxDelay: 5 })
+    const first = new KVOutbox(store, send, firstRetry)
+    await first.push({ type: 'write', entries: [['a', 1]] })
+    await first.push({ type: 'remove', keys: ['a'] })
+    expect(first.size).toBe(2)
+    expect(first.pending.has('a')).toBe(true)
+
+    firstRetry.stop() // the first session ends...
+    await first.replay() // ...once its in-flight replay attempt has failed
+    // A new session over the same store picks up the queue.
+    const second = new KVOutbox(store, send, new RetryTimer({ baseDelay: 5, maxDelay: 5 }))
+    online = true
+    await second.load()
+    await vi.waitFor(() => expect(second.size).toBe(0))
+    expect(sent).toEqual([
+      { type: 'write', entries: [['a', 1]] },
+      { type: 'remove', keys: ['a'] },
+    ])
+    expect(await store.readMany(['~q/o/000000000001', '~q/o/000000000002'])).toEqual([undefined, undefined])
+  })
+})