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/utils/extended-json.test.ts

@@ -0,0 +1,32 @@
+import { describe, expect, it } from 'vitest'
+import fc from 'fast-check'
+import { ExtendedJson } from '../../src/utils/extended-json.js'
+
+describe('ExtendedJson', () => {
+  it('round-trips bigint and Uint8Array, nested anywhere', () => {
+    const value = { a: 1n, b: [new Uint8Array([0, 1, 255]), -(2n ** 70n)], c: { d: new Uint8Array(0), e: 'plain', f: null } }
+    expect(ExtendedJson.stringify({ n: 5n, bytes: new Uint8Array([171, 205]) })).toBe('{"n":"<bigint: 5n>","bytes":"<uint8array: abcd>"}')
+    expect(ExtendedJson.parse(ExtendedJson.stringify(value))).toEqual(value)
+  })
+
+  it('round-trips arbitrary data (property)', () => {
+    const data = fc.letrec((tie) => ({
+      value: fc.oneof(
+        { depthSize: 'small' },
+        fc.bigInt(),
+        fc.uint8Array(),
+        fc.string(),
+        fc.double({ noNaN: true, noDefaultInfinity: true }),
+        fc.boolean(),
+        fc.constant(null),
+        fc.array(tie('value')),
+        fc.dictionary(fc.string(), tie('value')),
+      ),
+    })).value
+    fc.assert(
+      fc.property(data, (v) => {
+        expect(ExtendedJson.parse(ExtendedJson.stringify(v))).toEqual(v)
+      }),
+    )
+  })
+})