Ensure opaque paths always roundtrip

[domenic]

Implement whatwg/url#844

Roll web platform tests to web-platform-tests/wpt@048018b.

[domenic]

Implementation report: this was also a pretty quick task (9 minutes!). I think AI might have been able to speed it up, but not significantly, so I didn't bother trying to use it.

In retrospect I should have grouped all the non-TR46 spec updates.