Signed container, some kind of analogue to ASN.1-based CMS SignedData.
=> CMS

* Ability to embed the data in the signed container
* Ability to create detached signature
* Ability to use non-prehashed signature of the embedded data, potentially
  gaining more security
* Prehashed format is streaming friendly
* You can use [cm/hashed/Merkle] hashing mode to parallelise calculations
* Ability to attach arbitrary additional data
* Ability to store multiple signatures

Stored in a file, it should begin with "cm/signed" [encoding/MAGIC],
unless it is a [cm/pub/]lic key.

    schema-include av.tcl
    schema-include fpr.tcl
    
    signed {
        {field . {map}}
        {field load {with load}}
        {field sigs {list} {of sig} >0 optional}
        {field pubs {list} {of type map} >0 optional}
    }
    
    load {
        {field . {map}}
        {field t {str} >0}
        {# field v is optional, arbitrary type}
    }
    
    sig {
        {field . {map}}
        {field tbs {with tbs}}
        {field sign {with av}}
    }
    
    tbs {
        {field . {map}}
        {field sid {with fpr}}
        {field nonce {bin} >0 optional} {# random bytes}
        {field when {tai} utc prec=ms optional}
    
        {# recipient's fingerprints}
        {field encrypted-to {list} {of fpr} >0 optional}
    }
    av {
        {field . {map}}
        {field a {str} >0} {# algorithm identifier}
        {field v {bin}}
    }
    fpr {{field . {bin} len=32}}

Signature is created by signing the:

    [detached-data] || /load || /sig/./tbs

If no "/load/v" is provided, then the data is detached from the
"cm/signed" structure itself and is fed into hasher before that
structure. You can provide it any way you wish, but for keeping
that detached data closely to the "cm/signed", you should use the
following approach:

    prehash || BLOB(detached-data) || cm/signed

    prehash {
        {field . {map}}
        {field t {str} =prehash}
        {field algos {set} >0} {# set of hash algorithm identifiers}
    }

With "prehash" you initialise your hashers used during signing process
and feed BLOB's contents (not the encoded BLOB itself!) into the them.
prehash'es /algos must contain /sigs/*/sign/a identifiers:

"/sigs/*/tbs/when" is optional signing time.

Additional values that must be protected (covered by signature) are
placed in "/sigs/*/tbs" map. Non-protected (informational) fields
are placed outside it.

"/pubs" are optionally provided [cm/pub/]lic keys to help creating the
whole verification chain. They are placed outside "/sigs", because some
of them may be shared among signers.

If signed data is also intended to be [cm/encrypted/], then
"/sigs/*/tbs/encrypted-to" should be set to corresponding recipient's
public key fingerprint(s).

        Backlinks:
  0 [cm/]                                                              0
  1 [cm/hashed/]                                                       0
  2 [cm/pub/]                                                          0
  3 [cm/signed/ed25519-blake2b]             2025-06-09 07:24:59       15
  4 [cm/signed/ed25519ph-blake2b-merkle]    2025-06-09 07:24:59        6
  5 [cm/signed/gost3410]                    2025-06-26 13:25:25        9
  6 [cm/signed/gost3410-merkle]             2025-06-09 07:24:59        7
  7 [cm/signed/slh-dsa-shake-256s]          2025-06-24 12:41:34        8
  8 [cm/signed/slh-dsa-shake-256s-merkle]   2025-06-24 12:41:34        6