feat(xmldsig): add VerifyResult status model (P1-023)

[polaz]

Summary

Implement ROADMAP task P1-023: introduce VerifyResult with explicit DsigStatus, per-reference ReferenceResult, fail-fast semantics, and FailureReason classification.

Scope

• Add result/status types for XMLDSig verification outcome modeling
• Preserve per-reference diagnostics and first-failure index
• Keep fail-fast behavior on first digest mismatch
• Add tests for status/failure classification paths

Acceptance Criteria

• Verification API returns a structured status enum and failure reason enum
• Per-reference results are preserved in the result object
• First digest mismatch is represented explicitly (fail-fast)
• Unit/integration tests cover success, digest mismatch, and pipeline error paths

Estimate

1d

Ref: arch/xmldsig.md, .refs/features/03-xmldsig-verify.md

[coderabbitai]

🔗 Related PRs

#9 - feat(xmldsig): add URI dereference for Reference elements [merged]
#14 - feat(xmldsig): enveloped signature transform and pipeline executor [merged]
#27 - feat(xmldsig): add ecdsa signature verification [merged]
#30 - feat(xmldsig): add full signature verify pipeline [merged]
#32 - feat(xmldsig): add VerifyContext builder policies [merged]

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