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

src/utils/errors.ts

@@ -17,3 +17,11 @@ export class InvalidKeyError extends TypeError {
     this.name = 'InvalidKeyError'
   }
 }
+
+/** A remote storage could not be reached (network failure or 502/503/504). Writes may be retried later. */
+export class StorageUnavailableError extends Error {
+  constructor(message: string, options?: { cause?: unknown }) {
+    super(message, options)
+    this.name = 'StorageUnavailableError'
+  }
+}