feat: add tokenization infrastructure for improved line breaking

[nguillot]

Summary

This PR introduces a comprehensive tokenization infrastructure that lays the foundation for improved line breaking in mathematical expressions. The new architecture separates the concerns of tokenization, width calculation, and display generation, making the codebase more maintainable and enabling future line-fitting improvements.

Key Changes

New Tokenization System

• MTBreakableElement: Core data structure representing breakable units with break rules and penalties
• MTAtomTokenizer: Converts atoms into breakable elements with full support for 22 atom types
• MTElementWidthCalculator: Provides accurate width measurement for elements
• MTDisplayPreRenderer: Handles pre-rendering of complex atoms
• MTDisplayGenerator: Generates final display output from fitted elements
• MTLineFitter: Implements line fitting logic with break point selection

Refactoring

• Significantly simplified MTTypesetter.swift (reduced from ~2100 to ~400 lines)
• Separated tokenization logic into dedicated extension (MTTypesetter+Tokenization.swift)
• Updated MTMathUILabel to integrate with new tokenization system

Test Coverage

• Added 9 new test files with comprehensive coverage:
  • MTAtomTokenizerTests (311 lines)
  • MTBreakableElementTests (228 lines)
  • MTDisplayGeneratorTests (128 lines)
  • MTDisplayPreRendererTests (195 lines)
  • MTElementWidthCalculatorTests (169 lines)
  • MTLineFitterTests (203 lines)
  • MTTokenizationImprovementTests (145 lines)
  • MTTokenizationRealWorldTests (334 lines)
  • And more...
• Updated existing tests to work with new architecture
• Added regression tests for limit operators and relation operator spacing

Impact

• Code Quality: Better separation of concerns and reduced complexity
• Maintainability: Easier to understand and modify line-breaking logic
• Foundation: Enables future implementation of advanced line-fitting algorithms (e.g., greedy packing, Knuth-Plass)
• Test Coverage: Extensive test suite ensures reliability