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-in-memory.test.ts

@@ -76,3 +76,12 @@ describe('model-based: random writes vs a reference map', () => {
     )
   })
 })
+
+describe('child nodes', () => {
+  it('are frozen, so implementations cannot accidentally shadow shared state', () => {
+    const child = new KVStorageInMemory().child('a').child('b')
+    expect(Object.isFrozen(child)).toBe(true)
+    expect(() => ((child as unknown as { state: number }).state = 1)).toThrow(TypeError)
+    expect(child.key).toBe('a/b')
+  })
+})