Private key container.

    magic {{field . {magic} =cm/prv/v0}}
    
    schema-include algo-and-value.tcl
    schema-include fpr.tcl
    
    prv {
        {field . {map} len=~}
        {field prv {with algo-and-value}}
        {field pub-id {with fpr} optional}
    }
    -- schemas/prv-algos.tcl --
    prv-algos {
        {field . {choice}}
        {field ed25519-blake2b {bin} len=32}
        {field gost3410-256A {bin} len=32}
        {field gost3410-512C {bin} len=64}
        {field mceliece6960119-x25519 {bin} len=[expr {13948 + 32}]}
        {field sntrup761-x25519 {bin} len=[expr {1763 + 32}]}
        {field slh-dsa-shake-256s {bin} len=128}
    }

Stored in a file, it should begin with "cm/prv/v0" [encoding/MAGIC].

Optional /pub-id contains corresponding [cm/pub/]lic key's /data/id
to ease their searching.

        Backlinks:
  0 [cm/]                                                              0
  1 [cm/prv/ed25519-blake2b]                2026-05-18 15:50:39        4
  2 [cm/prv/gost3410]                       2026-05-18 15:50:39        5
  3 [cm/prv/mceliece6960119-x25519]         2026-05-18 15:50:39        4
  4 [cm/prv/slh-dsa-shake-256s]             2026-05-18 15:50:39        3
  5 [cm/prv/sntrup761-x25519]               2026-05-18 15:50:39        4