EC keygen now returns proper SPKI DER instead of raw point bytes.
RSA keygen uses caller-supplied exponent via new_with_exp() and
validates key size to 2048/3072/4096. Keybox parser extracts leaf
subject DN (not issuer). Added AttestKey=7 to KeyPurpose. Realigned
error variants with spec.
Splits concatenated DER cert chains into individual certificates,
extracts leaf issuer DN and notAfter via x509-cert crate. Handles
multi-byte DER length encoding (0x81-0x84).