Run WKWebView test harnesses invisibly without throttled rAF

Agent-driven WKWebView harnesses (swift script with AppKit) that bring their window to the front (level .floating, orderFrontRegardless) pop up on the user's screen. Keep them invisible: put the NSWindow far off-screen (setFrameOrigin(-30000,-30000)), alphaValue 0, ignoresMouseEvents true, NSApplication activation policy .prohibited, and never orderFront/activate. takeSnapshot still works. For frame-timing benchmarks, WebKit treats an off-screen window as occluded and throttles requestAnimationFrame. Disable occlusion detection in the harness only (private KVC _windowOcclusionDetectionEnabled = false on the WKWebView) rather than showing the window, and sanity-check once against an on-screen run.