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.

Sources/SwiftMath/MathRender/MTMathListBuilder.swift

@@ -702,6 +702,34 @@ public struct MTMathListBuilder {
             frac.numerator = self.buildInternal(true)
             frac.denominator = self.buildInternal(true)
             return frac;
+        } else if command == "dfrac" {
+            // Display-style fraction command has 2 arguments
+            let frac = MTFraction()
+            let numerator = self.buildInternal(true)
+            let denominator = self.buildInternal(true)
+
+            // Prepend \displaystyle to force display mode rendering
+            let displayStyle = MTMathStyle(style: .display)
+            numerator?.insert(displayStyle, at: 0)
+            denominator?.insert(displayStyle, at: 0)
+
+            frac.numerator = numerator
+            frac.denominator = denominator
+            return frac;
+        } else if command == "tfrac" {
+            // Text-style fraction command has 2 arguments
+            let frac = MTFraction()
+            let numerator = self.buildInternal(true)
+            let denominator = self.buildInternal(true)
+
+            // Prepend \textstyle to force text mode rendering
+            let textStyle = MTMathStyle(style: .text)
+            numerator?.insert(textStyle, at: 0)
+            denominator?.insert(textStyle, at: 0)
+
+            frac.numerator = numerator
+            frac.denominator = denominator
+            return frac;
         } else if command == "binom" {
             // A binom command has 2 arguments
             let frac = MTFraction(hasRule: false)
@@ -1048,6 +1076,34 @@ public struct MTMathListBuilder {
             frac.numerator = self.buildInternal(true)
             frac.denominator = self.buildInternal(true)
             return frac
+        } else if command == "dfrac" {
+            // Display-style fraction command has 2 arguments
+            let frac = MTFraction()
+            let numerator = self.buildInternal(true)
+            let denominator = self.buildInternal(true)
+
+            // Prepend \displaystyle to force display mode rendering
+            let displayStyle = MTMathStyle(style: .display)
+            numerator?.insert(displayStyle, at: 0)
+            denominator?.insert(displayStyle, at: 0)
+
+            frac.numerator = numerator
+            frac.denominator = denominator
+            return frac
+        } else if command == "tfrac" {
+            // Text-style fraction command has 2 arguments
+            let frac = MTFraction()
+            let numerator = self.buildInternal(true)
+            let denominator = self.buildInternal(true)
+
+            // Prepend \textstyle to force text mode rendering
+            let textStyle = MTMathStyle(style: .text)
+            numerator?.insert(textStyle, at: 0)
+            denominator?.insert(textStyle, at: 0)
+
+            frac.numerator = numerator
+            frac.denominator = denominator
+            return frac
         } else if command == "binom" {
             let frac = MTFraction(hasRule: false)
             frac.numerator = self.buildInternal(true)