parser|serial|tests: add W3C C14N conformance harness with c14n + parser fixes

[williamareynolds]

Summary

This corrects a minor issue with Parser/Serializer C14N standards to properly support #FIXED and #DEFAULT.

Type of Change

• Bug fix (corrects an existing issue)
• New feature (adds new functionality)
• Conformance fix (aligns behavior with the W3C spec or libxml2)
• Refactoring (no functional change)
• Documentation (docs, comments, examples)
• CI / tooling (build scripts, workflows, hooks)

Spec Reference

• Spec: XML 1.0
• Section: 3.3.2 Attribute Defaults

Test Plan

Added W3C C14N conformance tests.

• Added unit tests
• Added roundtrip test (parse -> serialize -> parse -> compare)
• Added regression test for the bug being fixed
• Verified against W3C Conformance Test Suite
• Tested manually with xmllint

Checklist

• Code is formatted (cargo fmt --all)
• No clippy warnings (cargo clippy --all-targets --all-features -- -D warnings)
• All tests pass (cargo test --all-features)
• Documentation updated for any public API changes
• Commit messages follow the <module>: <summary> convention
• No new dependencies added (or discussed and approved in an issue)

[williamareynolds]

Please hold, adding roundtrip test

Ready!