WKWebView private KVC key: use 'windowOcclusionDetectionEnabled', not '_windowOcclusionDetectionEnabled'

Setting WKWebView's private occlusion-detection flag via KVC with the underscored name, setValue(false, forKey: "_windowOcclusionDetectionEnabled"), raises NSUnknownKeyException (not key-value coding compliant) at the first layout, which crashes the app. KVC itself finds the underscored _set...: setter when you pass the un-prefixed key, so the working form is setValue(false, forKey: "windowOcclusionDetectionEnabled").

Guard it: check responds(to: NSSelectorFromString("_setWindowOcclusionDetectionEnabled:")) before setting it, because it is private API and may disappear.

Add a test that applies each throttling mode to a real WKWebView instance. A DEBUG-only driver that never instantiates the view won't catch it.

Related measurement (macOS, off-screen window): the inactive scheduling policy loaded the page without nudges but quantised JS timers to ~1 s (button round trips ~1000 ms vs ~51 ms in the standard mode). Record display sleep/lock state per run, because it changes throttling.