Function challenge_memory_key 

pub fn challenge_memory_key(domain: &str, target_class: TargetClass) -> String

Build a stable, lower-cased cache key for the challenge memory entry keyed by (domain, target_class).

Example

use stygian_charon::challenge_feedback::challenge_memory_key;
use stygian_charon::types::TargetClass;

let key = challenge_memory_key("Example.COM", TargetClass::Api);
assert!(key.starts_with("charon:challenge:example.com:"));