Function ref_compress 

pub fn ref_compress(
    cv: &[u32; 8],
    block: &[u32; 16],
    counter: u64,
    block_len: u32,
    flags: u32,
) -> [u32; 8]

Pure-Rust BLAKE3 compression, matching the in-circuit compression exactly.

• Exposed for callers that use a raw 2-to-1 compression as a tweakable hash.
• It reproduces the same value off-circuit for witness generation.