feat: support standard LaTeX relative sizes

[Happypig375]

Summary

• support \tiny, \scriptsize, \footnotesize, \small, \normalsize, \large, \Large, \LARGE, \huge, and \Huge in formula and text parsing
• scope declarations to their current group and preserve transitions through canonical serialization
• apply documented 10pt-class ratios from the painter's external base size across glyphs, rules, scripts, delimiters, radicals, accents, tables, and mixed text/math
• keep script scaling single-application and preserve existing absolute \fontsize{size}{content} precedence
• expose the ratio map read-only and represent text declarations structurally

Validation

• Core parser/structure focus: 17/17
• Core geometry focus: 17/17
• Rendering.Text parsing and real layout geometry: 16/16
• focused SkiaSharp/Avalonia decoded-pixel cases: 6/6
• Core and Rendering Release builds: 0 warnings/errors
• git diff --check
• independent review: approved

Closes #311