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-lock.json

@@ -9,6 +9,7 @@
       "version": "0.1.0",
       "devDependencies": {
         "@types/node": "^22.20.5",
+        "fake-indexeddb": "^6.2.6",
         "fast-check": "^4.10.2",
         "prettier": "^3.9.10",
         "typescript": "^5.9.3",
@@ -16,14 +17,6 @@
       },
       "engines": {
         "node": ">=20"
-      },
-      "peerDependencies": {
-        "vitest": ">=3"
-      },
-      "peerDependenciesMeta": {
-        "vitest": {
-          "optional": true
-        }
       }
     },
     "node_modules/@esbuild/aix-ppc64": {
@@ -1136,6 +1129,16 @@
         "node": ">=12.0.0"
       }
     },
+    "node_modules/fake-indexeddb": {
+      "version": "6.2.6",
+      "resolved": "https://registry.npmjs.org/fake-indexeddb/-/fake-indexeddb-6.2.6.tgz",
+      "integrity": "sha512-ihhmr0KoET51jT8R4YhbUYuLLvr1qqO6INISygtN16i8loR8kAxtAUr4+EKHTSMjL+lggaAn4OC7DRqzrNgnuQ==",
+      "dev": true,
+      "license": "Apache-2.0",
+      "engines": {
+        "node": ">=18"
+      }
+    },
     "node_modules/fast-check": {
       "version": "4.10.2",
       "resolved": "https://registry.npmjs.org/fast-check/-/fast-check-4.10.2.tgz",