Runtime: IntersectionObserver-based loader

TODO:

- [x] Decide whether to base it on the `intersect-resources` experiment or an alternative route.
- [x] Reimplement applySizeAndMedia
- [x] Reimplement display-observer for pause/resume
- [x] Reimplement ini-load measurement
- [ ] Reimplement resizer
- [x] Deferred builder

Start by reviewing the IntersectionObserver-based loader work described in this issue and identify the existing resizer implementation that remains unchecked. Reimplement the resizer behavior, then verify that the loader's completed checklist items still work and that resizer behavior is covered by the project's relevant tests.

Written by the indexing model from the issue text.