Ingest STIG data from DISA XCCDF/XML source

[rdwj]

Parse DISA's official STIG XCCDF XML to extract finding IDs, titles, descriptions, severity, and associated CCI/NIST controls. This ensures Stigcode's mapping data stays in sync with STIG releases.

Acceptance Criteria:

• Parse XCCDF benchmark XML for Application Security and Development STIG
• Extract: finding ID (V-XXXXXX), rule ID, title, description, severity category, fix text, CCI references
• Resolve CCI references to NIST 800-53 controls
• Store parsed data as structured YAML/JSON for use by the mapping engine
• stigcode stig import --xccdf <benchmark.xml> to ingest new STIG versions
• Diff capability to show what changed between STIG versions