Add display model

Sources/SwiftUIMath/Internal/Display/DisplayFraction.swift

@@ -0,0 +1,60 @@
+import CoreGraphics
+import Foundation
+
+extension Math {
+  final class DisplayFraction: DisplayNode {
+    var numerator: DisplayList?
+    var denominator: DisplayList?
+
+    var numeratorUp: CGFloat = 0 { didSet { updateNumeratorPosition() } }
+    var denominatorDown: CGFloat = 0 { didSet { updateDenominatorPosition() } }
+    var linePosition: CGFloat = 0
+    var lineThickness: CGFloat = 0
+
+    init(numerator: DisplayList?, denominator: DisplayList?, position: CGPoint, range: NSRange) {
+      self.numerator = numerator
+      self.denominator = denominator
+      super.init()
+      self.position = position
+      self.range = range
+    }
+
+    override var ascent: CGFloat {
+      get { (numerator?.ascent ?? 0) + numeratorUp }
+      set { super.ascent = newValue }
+    }
+
+    override var descent: CGFloat {
+      get { (denominator?.descent ?? 0) + denominatorDown }
+      set { super.descent = newValue }
+    }
+
+    override var width: CGFloat {
+      get { max(numerator?.width ?? 0, denominator?.width ?? 0) }
+      set { super.width = newValue }
+    }
+
+    override var position: CGPoint {
+      didSet {
+        updateDenominatorPosition()
+        updateNumeratorPosition()
+      }
+    }
+
+    private func updateDenominatorPosition() {
+      guard let denominator else { return }
+      denominator.position = CGPoint(
+        x: position.x + (width - denominator.width) / 2,
+        y: position.y - denominatorDown
+      )
+    }
+
+    private func updateNumeratorPosition() {
+      guard let numerator else { return }
+      numerator.position = CGPoint(
+        x: position.x + (width - numerator.width) / 2,
+        y: position.y + numeratorUp
+      )
+    }
+  }
+}