Load-failed class: capture visibilityState + nav-in-flight at throw time

Client unhandled-error handler ('Load failed' class) captures nothing about page state at throw time -- no document.visibilityState, no nav/unload-in-flight flag. Capture both at throw time so a future instance can be classified nav-away/backgrounding-abort vs genuine drop on evidence instead of inference. Additive to detail, not a suppression change. Found triaging id=5cc0de1a (2026-08-15, /mi-cursada), frequency baseline: 14 Load-failed rows/30d across 7 routes, 3 on /mi-cursada.