feat(style): the arithmetic behind a type scale

Quarter-point rounding, not whole points. The base size is 12.75pt, so rounding
a derived role to whole points would move it further than the step between two
adjacent base sizes -- the scale would quantise away the difference it exists to
express.

The floor is what keeps the meta role legible when someone runs a small base
size; below about seven points a timestamp stops being readable at any display
scale.

HellionChat/Ui/StyleEngine/TypeScaleMath.cs

@@ -0,0 +1,13 @@
+namespace HellionChat.Ui.StyleEngine;
+
+// TEST-MIRROR: Ui/TypeScaleMathTests.cs
+//
+// Rounds to quarter points rather than whole ones. The base size itself is
+// 12.75pt, so whole-point rounding would move a role by more than the step
+// between two adjacent base sizes -- the scale would quantise away the very
+// difference it exists to express.
+internal static class TypeScaleMath
+{
+    internal static float Resolve(float basePt, float factor, float minPt) =>
+        MathF.Max(minPt, MathF.Round(basePt * factor * 4f) / 4f);
+}