Implement accessibility rule: frame-tested

[koddsson]

Summary

• Implement the frame-tested rule so <iframe> and <frame> elements are confirmed to load the accessibility scanner script when scanning (Critical impact).
• Add rule metadata consistent with the README (review-item, best-practice tags) and ensure it integrates with the scanner pipeline.

Acceptance Criteria

• Add src/rules/frame-tested.ts implementing the rule logic and associated metadata.
• Add focused tests under tests/ that cover frames missing the script and frames where the script is present.
• Refresh documentation with npm run build:readme, and keep formatting/tests passing.

References

• README: frame-tested (Critical impact)
• Axe rule: https://dequeuniversity.com/rules/axe/4.4/frame-tested?application=RuleDescription

[koddsson]

I'm actually not sure if we need this rule just yet..