macOS Low Power Mode caps WKWebView requestAnimationFrame at 30 Hz

When benchmarking a WKWebView (even off-screen with occlusion detection and page-visibility throttling disabled, App Nap opted out), frame times suddenly read a steady 33 ms while JS render time stayed at 2-4 ms. Cause: the Mac was on battery with Low Power Mode on (pmset -g shows powermode 1), which pins rAF to 30 Hz. Check pmset before trusting frame numbers, and make rAF-paced tests time-based (e.g. bounds per elapsed ms) rather than assuming 60 frames per second; headless Chrome kept 16.7 ms frames on the same machine.