feat: add semantic text style axes

[Happypig375]

Summary

• introduce immutable semantic text-style axes for family, weight, posture, and capitals
• preserve legacy FontStyle compatibility while carrying multi-axis styles through parsing, cloning, equality, fusion, and typesetting
• serialize supported combinations canonically and reject unsupported slanted/small-caps combinations explicitly
• restore parser style scope on both successful and failed nested parses

Testing

• TextStyleTests: 17 passed
• LaTeXParserTest: 287 passed
• TextLaTeXParserTests: 241 passed
• Core and Rendering.Text builds passed
• scoped formatting verification and git diff --check passed

Stack

This PR is based on issue-311-relative-sizes / #314 because semantic text-style state builds on the relative-size model introduced there.

Closes #306