Update to roxygen2 8.1.0 to optimize package load time

[luwidmer]

roxygen2 now merges importFromdirectives since v8.1.0, see https://opensource.posit.co/blog/2026-08-04_roxygen2-8-1-0/ - this should improve package load time a bit as each importFrom() call adds up when loading the package.

[luwidmer]

This is in #520 now (since recompiling the docs for the Github checks requires this)