fix(core): guard element stamping to iframe-only (Studio preview)

packages/core/src/runtime/init.ts

@@ -982,7 +982,9 @@ export function initSandboxRuntimeModular(): void {
     });
     // Stamp data-start / data-duration on GSAP-targeted elements that lack
     // them so the Studio timeline can discover individual animated elements.
-    {
+    // Only when embedded in an iframe (Studio preview) — production renders
+    // run as the top-level page and must not mutate element timing.
+    if (window.parent !== window) {
       const rootComp = resolveRootCompositionElement();
       const rootDuration = boundDuration > 0 ? boundDuration : 0;
       const dur = String(rootDuration > 0 ? rootDuration : 1);