Enable ACT tests for td-headers-attr, th-has-data-cells

[koddsson]

Summary

• Enabled ACT tests for a25f45 (td-headers-attr) and d0f69e (th-has-data-cells)
• Bug fix in td-headers-attr.ts: Added self-reference check (cell headers must not reference themselves)
• Bug fixes in th-has-data-cells.ts:
  • Added buildColumnMap() to properly handle colspan/rowspan when determining column positions
  • Added explicit headers attribute override detection
  • Improved ARIA grid support with positional checking for role="columnheader"/"rowheader"
• 21 new ACT test files (12 for td-headers-attr, 9 for th-has-data-cells)
• All tests pass with no skips
• Updated rules.json to mark both as implemented

Part of #348

Test plan

• All 200 tests pass
• No regressions

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