Wire Rust accelerated backend to Python

Connect the new demean_accelerated module to Python and polish:
- Wire rust backend to use demean_accelerated instead of simple demean
- Fix MultiFE early convergence bug in 3+ FE demeaning
- Rename scatter/gather to apply_design_matrix for clarity
- Avoid per-column copy for Fortran-ordered input arrays
- Add type cast guard and #[inline(always)] on hot methods

Author: schroedk

.gitignore

@@ -42,4 +42,3 @@ coverage.xml
 # pixi environments
 .pixi/*
 !.pixi/config.toml
-benchmarks/results/