chore(xmldsig): introduce DsigError for verify pipeline

[polaz]

Summary

Implement ROADMAP task P1-024: add a public DsigError type for XMLDSig verification and align transform/c14n error layering with thiserror.

Scope

• Add DsigError as the top-level verify pipeline error type in xmldsig
• Keep C14nError and TransformError as lower-layer errors with typed source chaining
• Replace SignatureVerificationPipelineError public API usage with DsigError
• Update docs/tests/exports to match the new error naming and behavior

Acceptance Criteria

• Public verify APIs return Result<VerifyResult, DsigError>
• Existing failure categories remain preserved (missing elements, parse, transform, c14n, crypto, policy)
• Error conversions are explicit via #[from] where appropriate
• cargo clippy --all-targets --all-features -- -D warnings passes
• cargo nextest run --workspace and cargo test --doc --workspace pass

Estimate

1d

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

[coderabbitai]

🔗 Related PRs

#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]
#34 - feat(xmldsig): add VerifyResult status model [closed]

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