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.conformance.ts

@@ -71,6 +71,28 @@ export function runKVStorageConformance(name: string, make: () => BaseKVStorage
         expect(seen.filter((k) => k !== 'a/1')).toEqual(['10', '9', 'a', 'a/2', 'a0', 'b', 'c'])
       })
 
+      it('emits change events for writes and removals', async () => {
+        const events: unknown[] = []
+        const off = db.on('change', (event) => events.push(event))
+        await db.writeMany([['x', 1]])
+        await db.removeMany(['x'])
+        await db.writeIfAbsent([
+          ['a', 'no'],
+          ['y', 2],
+        ])
+        off()
+        expect(events).toEqual([
+          { changes: [{ key: 'x', value: 1 }], origin: 'local' },
+          { changes: [{ key: 'x' }], origin: 'local' },
+          { changes: [{ key: 'y', value: 2 }], origin: 'local' },
+        ])
+      })
+
+      it('stores bigint and Uint8Array values', async () => {
+        await db.writeMany([['bin', { n: 10n ** 20n, bytes: new Uint8Array([0, 255]) }]])
+        expect(await db.readMany(['bin'])).toEqual([{ n: 10n ** 20n, bytes: new Uint8Array([0, 255]) }])
+      })
+
       it('does not share references with callers', async () => {
         const [value] = (await db.readMany(['a/2'])) as [{ n: string }]
         value.n = 'changed'