Add display-style (dfrac) and text-style (tfrac) fraction commands
to SwiftMath's LaTeX parser. These commands force fractions to render
in specific styles regardless of context.
Implementation:
- Add dfrac parsing to prepend displaystyle to numerator/denominator
- Add tfrac parsing to prepend textstyle to numerator/denominator
- Implement in both parser functions in MTMathListBuilder.swift
Testing:
- Add testDisplayStyleFraction() for dfrac validation
- Add testTextStyleFraction() for tfrac validation
- Add testDisplayAndTextStyleFractions() for complex expressions
- All 180 tests pass on macOS and iOS simulator
Documentation:
- Update MISSING_FEATURES.md (7/12 features now implemented, 58%)
- Update README.md feature list to include dfrac and tfrac
Fixes issue where equations like y'=-\dfrac{2}{x^{3}} would fail to
parse with "Invalid command dfrac" error. This was blocking the
StepByStep feature preview rendering.
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