Function check_pins 

pub fn check_pins(
    username: &str,
    fetched_keys: &[(MurkRecipient, String)],
    pinned: &[String],
) -> Result<(), String>

Check fetched keys against pinned fingerprints.

Returns Ok(()) if pins match or no pins exist (TOFU). Returns Err with a description of what changed if pins don’t match.