feat: implement mapping integrity validator for CWE and MITRE ATLAS

[rashim27us]

Summary

This PR adds the first phase of automated cross-framework mapping integrity validation for the 2026 release, starting with CWE 4.20 and MITRE ATLAS 2026.06 as requested in #132.

• Adds a local and CI validator that checks mapping catalogs against version-pinned primary datasets, including identifier existence, duplicate IDs, title/metadata consistency, source version checks, pinned ATLAS source enforcement, and SHA-256 verification via new source_sha fields.
• Updates cwe-4.20.json and mitre-atlas-2026.06.json to pin machine-readable source URLs and record the verified source digests, replacing the CWE homepage URL with the version-specific XML archive.
• Adds offline negative tests, a GitHub Actions workflow, and mapping documentation so future framework validators can follow the same pattern.

Test plan

• Run python .github/scripts/test_validate_mappings.py
• Run python .github/scripts/validate_mappings.py --download
• Confirm both 2026/final/mappings/cwe-4.20.json and 2026/final/mappings/mitre-atlas-2026.06.json pass validation
• Verify negative cases fail for invalid IDs, duplicate IDs, title mismatches, deprecated ATLAS source paths, version mismatches, and SHA mismatches