Add font infrastructure

Sources/SwiftUIMath/Internal/Font/PlatformFont.swift

@@ -0,0 +1,35 @@
+@preconcurrency import CoreGraphics
+@preconcurrency import CoreText
+import Foundation
+
+extension Math {
+  final class PlatformFont: Sendable {
+    let font: Font
+    let cgFont: CGFont
+    let ctFont: CTFont
+    let metrics: FontMetrics
+
+    init?(font: Font) {
+      guard
+        let cgFont = FontRegistry.shared.graphicsFont(named: font.name),
+        let ctFont = FontRegistry.shared.font(named: font.name, size: font.size),
+        let table = FontRegistry.shared.table(named: font.name)
+      else {
+        return nil
+      }
+
+      self.font = font
+      self.cgFont = cgFont
+      self.ctFont = ctFont
+      self.metrics = FontMetrics(
+        font: font,
+        unitsPerEm: UInt(CTFontGetUnitsPerEm(ctFont)),
+        table: table
+      )
+    }
+
+    func withSize(_ size: CGFloat) -> PlatformFont {
+      PlatformFont(font: .init(name: font.name, size: size))!
+    }
+  }
+}