7/7: Structural elements & text alignment

[Amnell]

Summary

Add W3C coverage for structural elements and text alignment.

Changes

• Implement struct-cond-02-t
• Implement struct-cond-overview-02-f
• Fix struct-defs-01-t rendering
• Implement text-align-01-b

Merge order

⚠️ Merge these PRs in order: #8 → #9 → #10 → #11 → #12 → #13 → #14. After merging each PR, GitHub will automatically update the next PR's base to main.

Stack:

1. Swift 6 prep & test modernization (1/7: Swift 6 prep & test modernization #8)
2. Docs & tooling (2/7: Project docs, tooling & initial W3C coverage #9)
3. Pattern/gradient paint servers (3/7: Implement pattern & gradient paint servers #10)
4. Test reporting (4/7: HTML test report generation #11)
5. DOM scripting & currentColor (5/7: DOM scripting engine & currentColor hardening #12)
6. Shapes & coordinates (6/7: Shapes & coordinate rendering improvements #13)
7. Structural elements & text ← this PR