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-indexeddb.test.ts

@@ -0,0 +1,8 @@
+import 'fake-indexeddb/auto'
+import { KVStorageIndexedDB } from '../../src/kv-storage/kv-storage-indexeddb.js'
+import { runKVStorageConformance } from './kv-storage.conformance.js'
+
+let n = 0
+runKVStorageConformance('KVStorageIndexedDB', () => new KVStorageIndexedDB({ name: `db-${n++}` }))
+// Tiny batches exercise resuming scans across transactions.
+runKVStorageConformance('KVStorageIndexedDB (scan batches of 2)', () => new KVStorageIndexedDB({ name: `db-${n++}`, scanBatchSize: 2 }))