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/sync-helpers.ts

@@ -0,0 +1,35 @@
+import { KVStorageHttp } from '../../src/kv-storage/kv-storage-http.js'
+import { KVStorageHttpServer, type KVStorageHttpServerOptions } from '../../src/kv-storage/kv-storage-http-server.js'
+import { KVStorageInMemory } from '../../src/kv-storage/kv-storage-in-memory.js'
+import { KVStorageCounting } from '../helpers.js'
+
+/**
+ * An in-process server with a network switch. While `online` is false, new requests fail like a
+ * dropped connection (open SSE streams stay up, so pushes can still be simulated).
+ */
+export class TestNetwork {
+  online = true
+  readonly store = new KVStorageCounting(new KVStorageInMemory())
+  readonly server: KVStorageHttpServer
+  private readonly clients: KVStorageHttp[] = []
+
+  constructor(options: KVStorageHttpServerOptions = {}) {
+    this.server = new KVStorageHttpServer(this.store, options)
+  }
+
+  client(): KVStorageHttp {
+    const client = new KVStorageHttp({
+      url: 'http://server',
+      fetch: async (url, init) => {
+        if (!this.online) throw new TypeError('fetch failed')
+        return this.server.handle(new Request(url, init))
+      },
+    })
+    this.clients.push(client)
+    return client
+  }
+
+  async close(): Promise<void> {
+    await Promise.all(this.clients.map((c) => c.unwatch()))
+  }
+}