Function interface_hash 

pub fn interface_hash(signatures: &[&str]) -> u64

Compute the interface hash from a set of method signatures.

Signatures are sorted lexicographically before hashing to ensure order-independence. Each signature is joined with \n as a separator.

This function is not const because it allocates for sorting. The proc macro calls this at compile time via a build-script-like pattern, or uses fnv1a directly on pre-sorted, concatenated signatures.