feat: build X.509 certificate chains

[polaz]

Summary

Implement ROADMAP task P2-004: build ordered X.509 certificate chains from parsed KeyInfo/X509Data certificates.

Scope

• Parse and retain embedded X.509 certificates already extracted by P2-003.
• Identify the signing certificate by X509Data lookup hints where available: subject DN, issuer+serial, and SKI.
• Order certificates by issuer -> subject relationship with a bounded maximum depth.
• Preserve unsupported public-key algorithms as parsed certificate metadata without failing chain construction.
• Include dependency updates in the same PR before feature implementation.

Acceptance Criteria

• X509Data with unordered certificate entries produces an ordered chain starting at the signing certificate.
• SubjectName, IssuerSerial, and SKI hints select the expected signing certificate when present.
• Excessive depth, duplicate/ambiguous links, and missing issuer links are rejected or reported deterministically.
• Existing X509 parsing behavior remains compatible with current tests.
• Full workspace build, clippy, nextest, doc tests pass.

Estimate

3d implementation + tests, plus 0.5d dependency update/verification.

Closes: roadmap P2-004

[coderabbitai]

🔗 Related PRs

#30 - feat(xmldsig): add full signature verify pipeline [merged]
#32 - feat(xmldsig): add VerifyContext builder policies [merged]
#47 - feat(xmldsig): add KeyInfo dispatch parsing [merged]
#51 - feat(xmldsig): parse full x509data sub-elements [merged]
#53 - feat(xmldsig): parse X509Certificate with x509-parser [closed]

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