Function validate_csr_against_fleet_host 

pub fn validate_csr_against_fleet_host(
    csr_pubkey_raw: &[u8],
    declared_openssh_pubkey: Option<&str>,
) -> Result<()>

Bind agent identity to the host’s declared SSH host pubkey. Fail-closed: no pubkey in fleet.nix ⇒ enrollment refused (declarative-enrollment policy, no permissive fallback).