Function gemm_block 

pub fn gemm_block(a: MatrixBlock, b: MatrixBlock, c: &mut MatrixBlock)

Performs block-level GEMM: C = A * B + C for 64 x 64 bit blocks.

Arguments

• a - Left input block (64 x 64 bits)
• b - Right input block (64 x 64 bits)
• c - Accumulator block (64 x 64 bits)

For efficiency reasons, we mutate C in-place.

Implementation Selection

• x86_64 with AVX-512: Uses optimized assembly kernel
• Other platforms: Falls back to scalar implementation