Add support for dfrac and tfrac LaTeX commands

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.

MISSING_FEATURES.md

@@ -4,10 +4,10 @@ This document lists LaTeX features that are **not yet implemented** in SwiftMath
 
 ## Summary
 
-- **Total Features Tested**: 11
-- **Fully Implemented**: 6 (55%)
+- **Total Features Tested**: 12
+- **Fully Implemented**: 7 (58%)
 - **Partially Implemented**: 0 (0%)
-- **Not Implemented**: 5 (45%)
+- **Not Implemented**: 5 (42%)
 
 ---
 
@@ -126,6 +126,25 @@ x \, y \: z \; w                  % mixed spacing
 
 ---
 
+### 7b. ✅ `\dfrac` and `\tfrac` - Display/Text Style Fractions - **IMPLEMENTED**
+**Status**: ✅ Working
+**Description**: Fractions with forced display or text style
+
+**Test Results**: All tests passed
+- `\dfrac{1}{2}` - ✅ Works (display-style fraction)
+- `\tfrac{a}{b}` - ✅ Works (text-style fraction)
+- `y'=-\dfrac{2}{x^{3}}` - ✅ Works (complex expression)
+- Nested `\dfrac` and `\tfrac` - ✅ Works
+
+**Use Case**:
+- `\dfrac` forces display style (larger, more readable fractions)
+- `\tfrac` forces text style (smaller, inline fractions)
+- Useful when you want consistent fraction appearance regardless of context
+
+**Implementation**: Prepends style atoms to numerator and denominator to force rendering style.
+
+---
+
 ### 8. ❌ `\boldsymbol` - Bold Greek Letters
 **Status**: ❌ Not Implemented
 **Error**: `Invalid command \boldsymbol`