Add Define-XML 2.1 parser improvements

[PawanRamaMali]

Summary

The define_xml_to_evidence() parser handles Define-XML 2.0/2.1 but needs hardening for real-world variability.

Tasks

• Test against 5+ real-world Define-XML files from different sponsors
• Handle missing optional elements gracefully (e.g., missing <ComputationMethod>, missing <Origin>)
• Support Define-XML 2.1 extensions (value-level metadata, standards references)
• Add unit tests with synthetic Define-XML fixtures covering edge cases
• Return informative warnings for partially parseable files

Why

Define-XML files in the wild vary significantly. Robust parsing is critical for the Quality pillar scoring to be trustworthy.