Timing marks for RUM and benchmarks

When hydrateDusk has attached the app to the server's markup, it sets the User Timing mark dusk:hydrated and the measure dusk:hydration, which runs from navigation start (time 0) to that mark. This happens in development and in production. Both are a few microseconds of work and need no setup:

const [hydration] = performance.getEntriesByName("dusk:hydration", "measure");
reportTiming({ hydrationMs: hydration?.duration });

A page whose app did not mount (the global error document takes over) has no mark.