Function patch_seq_hmac_sha256 

#[unsafe(no_mangle)]
pub unsafe extern "C" fn patch_seq_hmac_sha256(stack: Stack) -> Stack

Compute HMAC-SHA256 of a message with a key

Stack effect: ( message key – String )

Returns the signature as a lowercase hex string (64 characters). Used for webhook verification, JWT signing, API authentication.

Safety

Stack must have two String values on top (message, then key)