Implement missing scanner rules with ACT test coverage

## Summary

15 scanner rules existed in `rules.json` but were not yet implemented. **13 of 15 are now done.**

## Remaining

| Scanner Rule | ACT Rule(s) | Blocker |
|---|---|---|
| `color-contrast` | `afw4f7` | Requires computed styles — not DOM-only feasible |

## Completed (13/15)

`avoid-inline-spacing`, `duplicate-id`, `aria-hidden-focus`, `object-alt`, `td-headers-attr`, `frame-focusable-content`, `html-has-lang`, `aria-required-children`, `th-has-data-cells`, `aria-required-parent`, `empty-heading`, `html-lang-valid`, `aria-command-name` (was already implemented, just needed marking)

Start by reading rules.json and the ACT rule mapping for color-contrast (afw4f7). Review the existing implementations and ACT coverage for the 13 completed rules, then determine whether the remaining rule can meet the scanner's DOM-only constraints; done means the rule is implemented and covered, or the blocker is documented clearly.

Written by the indexing model from the issue text.