Function dsig_chain_block_kernel 

pub fn dsig_chain_block_kernel(
    secret: [b8; 18],
    nonce: [b8; 16],
    index: b32,
    depth: b32,
) -> [b8; 64]

RHDL byte packing for the exact dsig_chain_block salt domain.

The index and depth are reduced modulo 2^32 by their input types; the caller must enforce WOTS protocol bounds independently. This function only packs the block and does not perform a hash or allocate a one-time key.

Inputs: the 18-byte node, the 16-byte nonce, and 32-bit index and depth. Output: the 64-byte chain block. Combinational: one 32-bit add-and-multiply-by-constant, the rest is wiring.