feat(playground): export Y.Doc from debug menu (#14893)

## Summary
- add an Export Y.Doc debug menu item
- encode the active store spaceDoc with Y.encodeStateAsUpdate
- download the update as a binary ydoc-update file

## Test
- yarn workspace @blocksuite/playground build

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **New Features**
* Added Y.Doc export functionality to the debug menu, enabling users to
download the current space document state as a binary update file
through the "Test Operations" → "Export" menu.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

blocksuite/playground/apps/_common/components/starter-debug-menu.ts

@@ -70,6 +70,7 @@ import { css, html } from 'lit';
 import { customElement, property, query, state } from 'lit/decorators.js';
 import * as lz from 'lz-string';
 import type { Pane } from 'tweakpane';
+import * as Y from 'yjs';
 
 import type { CommentPanel } from '../../comment/index.js';
 import { createTestEditor } from '../../starter/utils/extensions.js';
@@ -337,6 +338,14 @@ export class StarterDebugMenu extends ShadowlessElement {
     );
   }
 
+  private _exportYDoc() {
+    const encodeUpdate = Y.encodeStateAsUpdate(this.doc.spaceDoc);
+    const blob = new Blob([new Uint8Array(encodeUpdate)], {
+      type: 'application/octet-stream',
+    });
+    download(blob, 'ydoc-update');
+  }
+
   private _getStoreManager() {
     return this.editor.std.get(StoreExtensionManagerIdentifier);
   }
@@ -834,6 +843,9 @@ export class StarterDebugMenu extends ShadowlessElement {
                   <sl-menu-item @click="${this._exportSnapshot}">
                     Export Snapshot
                   </sl-menu-item>
+                  <sl-menu-item @click="${this._exportYDoc}">
+                    Export Y.Doc
+                  </sl-menu-item>
                 </sl-menu>
               </sl-menu-item>
               <sl-menu-item>