Fix boosted history scroll restoration

[scriptogre]

Bug

hx-boosted back/forward navigation loses scroll position.

The browser restores scroll against the old DOM. The position clamps before htmx swaps the restored page.

Reproduce

1. Open https://four.htmx.org/reference.
2. Scroll to the bottom.
3. Press e.g. <hx-partial>.
4. Go back.

Fix

Modern browsers:

1. Intercept traversal with the Navigation API.
2. Swap the restored content.
3. Let the browser restore scroll.

Older browsers:

1. Save scroll when pushing the URL.
2. Swap the restored content.
3. Restore the saved position.

The fallback covers common back and forward cases.

Full native behavior needs more code (track scroll, update history entries, handle page lifecycle, restore after popstate, handle repeated visits).

Prior work

PR #3704 only fixed the rare history: "reload" case and was more complex.

Analysis: https://gist.github.com/scriptogre/40c5c4af3ba971b7cf9be10603a80e7c