mdoc M1: CBOR/COSE foundation

[DibranMulder]

Implements M1 — CBOR/COSE foundation from the mdoc implementation plan (docs/mdoc-implementation-plan.md).

Closes #563. Part of #562.

What this adds

A new self-contained package eudi/credentials/mdoc/, sibling of sdjwtvc/, with the low-level primitives every later mdoc milestone builds on. No dependencies on the OpenID4VCI/VP packages.

Dependencies (go.mod)

• github.com/fxamacker/cbor/v2 — different import path than the existing v1, so both coexist; v2 is required for proper tag-24 and deterministic encoding support.
• github.com/veraison/go-cose — COSE_Sign1.

cbor.go

• Fixed canonical CBOR encode mode (length-first map ordering, definite lengths, shortest-form ints) and a decode mode that rejects indefinite-length items, per ISO/IEC 18013-5 §9.1.2.
• EncodedCBOR — the tag 24 #6.24(bstr .cbor X) wrapper, preserving embedded bytes verbatim for byte-exact digesting/signing.
• FullDate — tag 1004 (RFC 8943) YYYY-MM-DD.
• DateTime — tag 0 (RFC 8949 §3.4.1) RFC 3339, normalised to UTC with no fractional seconds.

cose.go

• COSE_Key (EC2/P-256) ⇄ *ecdsa.PublicKey/PrivateKey ⇄ jwk.Key, with the RFC 9052 leading-zero coordinate padding.
• Sign1 / Verify1 — thin wrappers around go-cose for the untagged COSE_Sign1 objects mdoc uses for issuerAuth/deviceSignature, with x5chain (label 33) support.
• X5Chain — extract and parse the certificate chain from a COSE_Sign1.

Test vectors

Ported verbatim from the multipaz reference implementation (org.multipaz.mdoc.TestVectors), originally from ISO/IEC 18013-5:2021 Annex D, into testvectors_test.go.

Tests

• Deterministic-encoding byte-equality; tagged-type round trips + byte-exact golden checks (incl. the Annex D validityInfo signed value).
• COSE_Key ⇄ JWK ⇄ ecdsa round trips over many generated keys; coordinate padding; non-P-256 rejection.
• COSE_Sign1 sign/verify, x5chain encode/extract, tamper/wrong-key negatives.
• Cross-validation against ISO 18013-5 Annex D: extracts the issuerAuth COSE_Sign1 from the worked-example device response and verifies its signature against the Annex D document-signer certificate.
• A CBOR decode fuzz target (FuzzCOSEKeyToECDSAPublicKey), exercised for ~850k execs with no panics.

ok  github.com/privacybydesign/irmago/eudi/credentials/mdoc

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