Function mds_multiply 

pub fn mds_multiply<F, A, const WIDTH: usize>(
    state: &mut [A; WIDTH],
    matrix: &[[F; WIDTH]; WIDTH],
)
where
    F: PrimeCharacteristicRing,
    A: Algebra<F>,

Dense matrix-vector product, applied in place to a fixed-width state vector.

Overview

• Generic O(t^2) fallback for any dense square matrix.
• Circulant matrices have faster paths in this module (Karatsuba, FFT).
• Sparse or diagonal layouts can skip full-row scans entirely.

Arguments

• The state vector, overwritten with the product on return.
• The matrix, indexed row-first as m[row][col].

Performance

• Runtime: O(t^2) ring operations for a width-t state.
• Allocations: one stack snapshot of the input state.