Add comprehensive validation report

- Documents 8 test categories with 50+ scenarios
- Details WordPress plugin/theme compatibility testing (8+ plugins, 5+ themes)
- XSS prevention testing across 8+ attack vectors
- IndieWeb security (Micropub, Webmention, IndieAuth)
- Rate limiting validation
- Security headers compliance
- Execution instructions for manual, automated, and CI/CD runs
- Framework assessment and recommendations

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