test(xmldsig): implement P1-025 donor full verification suite

[polaz]

Summary: implement ROADMAP task P1-025 with donor full verification coverage for XMLDSig verify pipeline.

Scope:

• Verify supported algorithms from tests/fixtures/xmldsig/aleksey-xmldsig-01 via end-to-end verify pipeline.
• Add basic vectors from donors/xmlsec/tests/merlin-xmldsig-twenty-three (enveloped DSA as expected-skip, enveloping RSA, X509 cert-chain vectors).
• Track pass/fail/skip counts explicitly in test reporting.

Acceptance criteria:

• New integration tests provide deterministic pass/fail/skip accounting.
• RSA-SHA*, ECDSA-SHA* aleksey vectors pass.
• Merlin basic coverage exists with explicit skip reasons for unsupported algorithms/features.
• cargo clippy --all-targets --all-features -- -D warnings passes.
• cargo nextest run --workspace and cargo test --doc --workspace pass.

Estimate: 3d.

Ref: arch/ROADMAP.md P1-025; arch/xmldsig.md; .refs/features/03-xmldsig-verify.md

[coderabbitai]

🔗 Related PRs

#22 - feat(xmldsig): add RSA signature verification [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 [merged]

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