Function sig_len 

pub fn sig_len<P: Params>() -> usize

Compute the total SLH-DSA signature length in bytes for parameter set P.

The signature layout is R || SIG_FORS || SIG_HT, with total size: n + k*(1+a)*n + (h + d*len)*n.

This is a runtime function rather than a const due to Rust const-generics limitations.