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/helpers.ts

@@ -10,6 +10,7 @@ export async function collect<T>(iterable: AsyncIterable<T>): Promise<T[]> {
 /**
  * Wraps another KV storage and counts primitive calls. Also shows the wrapper pattern
  * (e.g. an encryption layer): extend BaseKVStorage and delegate the five primitives.
+ * Counts are reported under the public method names (writeMany, removeMany, writeIfAbsent).
  */
 export class KVStorageCounting extends BaseKVStorage {
   calls: Record<string, number> = {}
@@ -25,13 +26,13 @@ export class KVStorageCounting extends BaseKVStorage {
   readMany(keys: string[]) {
     return this.count('readMany', () => this.inner.readMany(keys))
   }
-  writeMany(entries: [string, unknown][]) {
+  protected applyWrite(entries: [string, unknown][]) {
     return this.count('writeMany', () => this.inner.writeMany(entries))
   }
-  removeMany(keys: string[]) {
+  protected applyRemove(keys: string[]) {
     return this.count('removeMany', () => this.inner.removeMany(keys))
   }
-  writeIfAbsent(entries: [string, unknown][]) {
+  protected applyWriteIfAbsent(entries: [string, unknown][]) {
     return this.count('writeIfAbsent', () => this.inner.writeIfAbsent(entries))
   }
   scan(options?: ScanOptions) {