Function engine_memory_key 

pub fn engine_memory_key(key: &EngineKey) -> String

Build a stable cache key for the challenge memory entry keyed by EngineKey.

The wire shape is charon:challenge:engine[/version][+tls_profile]:target_class so it round-trips with EngineKey’s Display impl and never collides with charon:pow:... (T93) or charon:token_nonce:... (T91) entries on a shared backing primitive.

Example

use stygian_charon::challenge_feedback::{engine_memory_key, EngineKey};
use stygian_charon::types::TargetClass;
use stygian_charon::vendor_classifier::VendorId;

let key = EngineKey {
    engine: VendorId::Cloudflare,
    version: None,
    target_class: TargetClass::Api,
    tls_profile: None,
};
let wire = engine_memory_key(&key);
assert!(wire.starts_with("charon:challenge:cloudflare:api"));