Add KVStorageComposite, IndexedDB storage and SSE change streams

- Every KV storage emits change events (base class owns writeMany/removeMany/
  writeIfAbsent; implementations provide protected apply* methods)
- KVStorageHttpServer streams changes over SSE (GET /events) with a resumable
  history; KVStorageHttp.watch() subscribes and resumes via Last-Event-ID
- KVStorageIndexedDB for local persistence
- KVStorageComposite keeps replicas of a source in sync: local reads, resume
  instead of full sync, offline outbox replayed in order, echo tracking
- Values may contain bigint/Uint8Array (Extended JSON over the network)
- Vendor sse-session, event-emitter and extended-json (no runtime deps)
- Freeze child nodes so shadowed state on children fails loudly
- CI on Node 22, 24 and 26

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

test/kv-storage/kv-storage-http-watch.test.ts

@@ -0,0 +1,61 @@
+import { afterEach, describe, expect, it } from 'vitest'
+import type { KVChangeEvent } from '../../src/kv-storage/base-kv-storage.js'
+import { StorageUnavailableError } from '../../src/utils/errors.js'
+import { TestNetwork } from './sync-helpers.js'
+
+let network: TestNetwork
+afterEach(() => network?.close())
+
+describe('KVStorageHttp.watch (SSE)', () => {
+  it('receives changes made by other clients as remote change events', async () => {
+    network = new TestNetwork()
+    const watcher = network.client()
+    await watcher.watch()
+    const received = watcher.waitFor('change', (e) => e.origin === 'remote')
+    await network.client().child('users').set('1', { name: 'Ada', n: 1n })
+    const event = await received
+    expect(event.changes).toEqual([{ key: 'users/1', value: { name: 'Ada', n: 1n } }])
+    expect(event.id).toMatch(/^\w+-1$/)
+    expect(watcher.lastEventId).toBe(event.id)
+  })
+
+  it('resumes from lastEventId and replays missed changes', async () => {
+    network = new TestNetwork()
+    const writer = network.client()
+    const watcher = network.client()
+    await watcher.watch()
+    const first = watcher.waitFor('change', (e) => e.origin === 'remote')
+    await writer.writeMany([['a', 1]])
+    await first
+    await watcher.unwatch()
+    await writer.writeMany([['b', 2]])
+    await writer.writeMany([['c', 3]])
+    const replayed: KVChangeEvent[] = []
+    watcher.on('change', (e) => replayed.push(e))
+    await watcher.watch() // resolves once caught up, so the missed changes have been emitted
+    expect(replayed.map((e) => e.changes)).toEqual([[{ key: 'b', value: 2 }], [{ key: 'c', value: 3 }]])
+  })
+
+  it('asks for a resync when the missed changes are no longer in the history', async () => {
+    network = new TestNetwork({ historySize: 1 })
+    const writer = network.client()
+    const watcher = network.client()
+    await watcher.watch()
+    const seen = watcher.waitFor('change', () => true)
+    await writer.writeMany([['a', 1]])
+    await seen
+    await watcher.unwatch()
+    await writer.writeMany([['b', 2]])
+    await writer.writeMany([['c', 3]])
+    const resync = watcher.waitFor('resync', () => true)
+    await watcher.watch()
+    expect(await resync).toEqual({ id: expect.stringMatching(/-3$/) })
+  })
+
+  it('reports an unreachable server as StorageUnavailableError', async () => {
+    network = new TestNetwork()
+    const client = network.client()
+    network.online = false
+    await expect(client.readMany(['x'])).rejects.toBeInstanceOf(StorageUnavailableError)
+  })
+})