Function compute_key_id 

pub fn compute_key_id(key_name: &str, pubkey: &[u8; 32]) -> [u8; 4]

Compute a C2SP key ID from a key name and Ed25519 public key.

key_id = SHA-256(key_name + "\n" + 0x01 + pubkey)[0..4]

Args:

• key_name — The log’s key name (e.g., “auths.dev/log”).
• pubkey — 32-byte Ed25519 public key.

Usage:

let key_id = compute_key_id("auths.dev/log", &pubkey_bytes);