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

src/document-storage/base-document-storage.ts

@@ -27,7 +27,7 @@ export type ChildDoc<S> = ValueOf<ChildOf<S, KeyOf<S>>>
  */
 export abstract class BaseDocumentStorage<S extends Spec = AnySpec> {
   /** Unescaped path of this node (`[]` for the root, which has no own document). */
-  path: readonly string[] = []
+  readonly path: readonly string[] = []
   protected readonly ids = new Ulid()
 
   abstract readMany(paths: (readonly string[])[]): Promise<(Doc | undefined)[]>
@@ -40,8 +40,11 @@ export abstract class BaseDocumentStorage<S extends Spec = AnySpec> {
   abstract query(parent: readonly string[], query: DocumentQuery): AsyncIterable<FindResult>
 
   child<K extends KeyOf<S>>(key: K): BaseDocumentStorage<ChildOf<S, K>> {
-    const node = Object.create(this) as BaseDocumentStorage<ChildOf<S, K>>
-    node.path = [...this.path, Keys.segment(key)]
+    const node = Object.create(this, { path: { value: [...this.path, Keys.segment(key)], enumerable: true } }) as BaseDocumentStorage<ChildOf<S, K>>
+    // Frozen: child nodes share the root's state through the prototype, so an implementation that
+    // reassigns a field (`this.x = ...`) while running on a child would silently shadow it.
+    // Keep mutable state in objects (Maps, a `state` object) instead; this makes mistakes throw.
+    Object.freeze(node)
     return node
   }