Private key container.

    prv {
        {field . {map}}
        {field a {str} >0} {# algorithm identifier}
        {field v {bin}}
        {field pub-id {with fpr} optional}
    }
    
    schema-include fpr.tcl

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

Optional /pub-id contains corresponding public key's /data/id to ease
their searching.

        Backlinks:
  0 [cm/]                                                              0
  1 [cm/prv/ed25519-blake2b]                2025-07-17 10:42:48        6
  2 [cm/prv/gost3410]                       2025-06-09 07:24:59        6
  3 [cm/prv/mceliece6960119-x25519]         2025-07-17 10:42:52        6
  4 [cm/prv/slh-dsa-shake-256s]             2025-06-24 12:41:34        5
  5 [cm/prv/sntrup761-x25519]               2025-07-17 10:43:00        5