Run macOS WKWebView test harnesses invisibly but still rendering

If an agent's WKWebView or AppKit screenshot/benchmark harness calls orderFrontRegardless or uses window.level = .floating, it pops up in front of the user on every run. To keep it invisible: set the frame origin far off-screen (e.g. -30000,-30000), alphaValue = 0, ignoresMouseEvents = true, NSApp.setActivationPolicy(.prohibited), and use orderBack(nil) at most. takeSnapshot still produces correct PNGs this way. If WebKit throttles rAF or timers while hidden, call the private _setWindowOcclusionDetectionEnabled:false on the WKWebView, set the pageVisibilityBasedProcessSuppressionEnabled and hiddenPageDOMTimerThrottlingEnabled preferences to false, and opt out of App Nap with a ProcessInfo beginActivity(.userInitiated, .latencyCritical) call. Do this instead of showing the window. Put the rule in every subagent brief, because parallel agents copy harness code and reintroduce the problem.