fix(sdk-playground): implement PreviewAdapter.attachSync no-op stub

PlaygroundPreview implements PreviewAdapter but was missing attachSync,
which this branch added to the interface — a real TS break (no
typecheck script wires sdk-playground into CI, so nothing caught it).
Mirrors the same no-op stub already added to HeadlessPreviewAdapter.

packages/sdk-playground/src/main.ts

@@ -85,6 +85,10 @@ class PlaygroundPreview implements PreviewAdapter {
       this.handlers = this.handlers.filter((h) => h !== handler);
     };
   }
+
+  attachSync(_comp: Composition): () => void {
+    return () => {};
+  }
 }
 
 // ── Preview iframe ────────────────────────────────────────────────────────────