Function chain 

pub fn chain<P: Params>(
    x: &[u8],
    i: u32,
    s: u32,
    pk_seed: &[u8],
    adrs: &mut Adrs,
) -> Vec<u8> 

Apply the WOTS+ chain function s times starting from step i.

Implements Algorithm 5 of FIPS 205. Computes F^s(X) by iterating the tweakable hash function F a total of s times, with hash addresses ranging from i to i + s - 1. This recursive implementation is provided for clarity; the internal signing and verification routines use chain_iter (private helper) for efficiency.

Returns an n-byte hash chain value.