Private key container.

    av {
        {field . {map}}
        {field a {str} >0} {# algorithm identifier}
        {field v {bin}}
    }

Stored in a file, it should begin with "cm/prv" [encoding/MAGIC].

        Backlinks:
  0 [cm/]                                                              0
  1 [cm/prv/ed25519-blake2b]                2025-09-29 08:25:34        6
  2 [cm/prv/gost3410]                       2025-09-29 08:25:34        6
  3 [cm/prv/mceliece6960119-x25519]         2025-09-29 08:25:34        6
  4 [cm/prv/slh-dsa-shake-256s]             2025-09-29 08:25:34        5
  5 [cm/prv/sntrup761-x25519]               2025-09-29 08:25:34        5