Playgrounds clips: advance animation clocks in child frames #516

Follow-up from the full Slop Cop review of #515 at 722f4837dda96ab00fb20e5df7c5ff4703ecf0e5 (P2).

The export injects its page clock into every frame via Page.addScriptToEvaluateOnNewDocument, but clipClock.advanceTo evaluates only the top-level window. An HTML figure using an animated iframe.srcdoc leaves the child's timers and requestAnimationFrame callbacks paused at zero. A parent action waiting for that child can also fail to finish.

Advance each active frame's clock through its own CDP execution context, preserving the existing opaque-origin sandbox and CSP. Verify with a srcdoc fixture whose timer and rAF marker advance alongside the parent in decoded clip frames.

The current fix covers top-level HTML and Anatomy figures; this optional child-frame case is tracked separately under the standing P0/P1-only review policy.

BB-Thread-ID: thr_wgcdsffskf