Host-side RSA-2048 key material for OpenPGP card import.
A small front-end helper shared by keyroostctl and keyroost. It owns the one
external-dependency exception in the workspace — the rsa crate — so the
protocol crates stay dependency-free and the security-critical keygen / parse
logic lives in exactly one place. It produces the full CRT component set
(e, p, q, u, dp, dq, n, minimal big-endian) that keyroost_openpgp's import
path frames into the card's Extended Header List; the card selects which of
those parts it actually wants per its declared algorithm attributes.