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.test.ts

@@ -9,7 +9,7 @@ import { runKVStorageConformance } from './kv-storage.conformance.js'
 import { serve } from './node-server.js'
 
 runKVStorageConformance('KVStorageHttp (in-process)', () => {
-  const server = new KVStorageHttpServer(new KVStorageInMemory(), '/kv')
+  const server = new KVStorageHttpServer(new KVStorageInMemory(), { basePath: '/kv' })
   return new KVStorageHttp({ url: 'http://test/kv', fetch: async (input, init) => server.handle(new Request(input, init)) })
 })
 
@@ -22,7 +22,7 @@ describe('over a real socket', () => {
   })
   afterAll(() => server.close())
   const fresh = () => {
-    stores.push(new KVStorageHttpServer(new KVStorageInMemory(), `/${stores.length}`))
+    stores.push(new KVStorageHttpServer(new KVStorageInMemory(), { basePath: `/${stores.length}` }))
     return new KVStorageHttp({ url: `${server.url}/${stores.length - 1}` })
   }