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

package.json

@@ -4,7 +4,7 @@
   "description": "Layered TypeScript storage: KV -> Events -> Documents",
   "type": "module",
   "engines": {
-    "node": ">=20"
+    "node": ">=22"
   },
   "main": "./dist/index.js",
   "types": "./dist/index.d.ts",
@@ -29,6 +29,7 @@
   },
   "devDependencies": {
     "@types/node": "^22.20.5",
+    "fake-indexeddb": "^6.2.6",
     "fast-check": "^4.10.2",
     "prettier": "^3.9.10",
     "typescript": "^5.9.3",