Hey, this is excellent work. Quite a bit to get right, thanks for the effort!

I only review the CLS changes, and I think there's a potentially cleaner way to do it. I didn't review the specifics of the other metrics since I the change would alter a lot there too, so wanted to discuss that first.

Basically:

• I think the observe() helper, which all metrics use, should be the only place where we look at navigationID and split entries whenever it changes. See: https://github.com/GoogleChrome/web-vitals/blob/main/src/lib/observe.ts#L52
• Whenever navigationID changes (or, the first time for initial nav), that helper can look up pageURL, etc, that way we have shared cache for all metrics
• Each metric implementation right now already uses helpers like onFCP, onHidden, onBFCacheRestore... in order to "finalize" and/or "reset" its metrics. I think we could have another helper like onNavigate(), which can be used to flush/reset metrics.

One tricky bit may be that the onNavigate() helper needs to be called before any new Entry that uses that navigationID, and you could have entries with different navigationID in a single PerformanceObserver callbacks, especially when using buffered entries.

That said, I think all this code made be fragile to such cases and needs careful testing.

Keep at it!