feat(xmldsig): build x509 certificate chains

[polaz]

Summary

• Implement P2-004 certificate chain building for parsed X509Data certificates.
• Add certificate serial metadata and ordered chain indexes starting at the selected signing certificate.
• Select signing certificate via X509SubjectName, X509IssuerSerial, or X509SKI hints, with deterministic ambiguity errors.
• Checked dependency updates in this PR scope: cargo update produced no tracked diff because Cargo.lock is untracked, but the workspace was verified against the resolved updated graph.

Testing

• cargo fmt -- --check
• cargo check --workspace
• cargo clippy --workspace --all-targets -- -D warnings
• cargo nextest run --workspace
• cargo test --doc --workspace
• cargo build --workspace

Closes #60

[coderabbitai]

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro Plus

Run ID: 8704669a-216e-49e0-82ab-c9d84b308615

📥 Commits

Reviewing files that changed from the base of the PR and between 4fb8503 and 199a7c1.

📒 Files selected for processing (1)

• src/xmldsig/parse.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/#60-x509-chain-building

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[greptile-apps]

Greptile Summary

This PR implements X.509 certificate chain building on top of the already-parsed X509Data certificates. It adds certificate_chain (an ordered index vector starting at the signing cert) and serial_number/serial_number_hex fields to ParsedX509Certificate, a decimal-to-hex serial converter for XMLDSig X509SerialNumber values, and signing-certificate selection via X509SubjectName, X509IssuerSerial, and X509SKI hints with deterministic ambiguity errors.

• Chain building walks issuer DNs, rejects cycles and depths > MAX_X509_CHAIN_DEPTH, and falls back to leaf detection when no explicit hint is present.
• The three issues raised in previous rounds — decimal-vs-hex serial mismatch, depth off-by-one, and DER sign-extension leading zero — are all addressed with targeted fixes and regression tests.

Confidence Score: 5/5

Safe to merge — the chain-building and signing-cert selection logic is sound, and all three issues from previous review rounds have been correctly resolved with regression tests.

The core algorithms (decimal-to-hex serial conversion, DER sign-extension stripping, leaf detection, cycle detection, depth guard) have been reviewed and are correct. The only remaining item is a provably unreachable top-of-loop depth check that became dead code after the pre-push guard was introduced — it cannot cause incorrect behavior.

No files require special attention.

Important Files Changed

Filename	Overview
src/xmldsig/parse.rs	Adds X.509 certificate chain building, signing-cert selection via Subject/IssuerSerial/SKI hints, serial number fields with DER sign-extension stripping, and a decimal-to-hex serial converter. Previously flagged issues (hex vs. decimal serial matching, depth off-by-one, DER leading zero) are all resolved. One unreachable top-of-loop depth guard remains as dead code.

_{Reviews (3): Last reviewed commit: "fix(xmldsig): canonicalize x509 serial v..." | Re-trigger Greptile}