Function multisig_satisfied 

pub fn multisig_satisfied(valid_signers: usize, k: usize) -> bool

A k-of-n multi-signature over the payload is satisfied iff at least k distinct valid signer attestations are present. The individual signatures are verified by the crypto layer (Ed25519 / post-quantum ML-DSA via fiduciary_crypto); this is the threshold gate. valid_signers = the count of distinct verified signers.