Function pick_or_plan_keygen 

pub fn pick_or_plan_keygen(ssh_dir: &Path, existing: &[String]) -> KeyPlan

Decide which client key host setup-key should use. PURE: existing is the set of candidate basenames the caller found actually present in ssh_dir — this only picks among them. Reuses the first preferred name present (ed25519 > ecdsa > rsa); plans to generate id_ed25519 when none exist. Never plans to overwrite an existing key.