feat(Graphics): add `fitScale` function

```fsharp
/// Вычисляет коэффициент масштаба, необходимый, чтобы вписать изображение в указанную область
let fitScale (viewportSize: Size) (imageSize: Size) =
let dw = imageSize.Width - viewportSize.Width
let dh = imageSize.Height - viewportSize.Height

if dw > dh then
if dw > 0 then
float viewportSize.Width / float imageSize.Width
else
1.0
else
if dh > 0 then
float viewportSize.Height / float imageSize.Height
else
1.0

// Assert.equal
// (
// let imageSize = Size.create 4 3
// let scale = fitScale (Size.create 3 4) imageSize
// imageSize * scale
// )
// (Size.create 3 2)
// ""
```

Start from the proposed fitScale definition and the commented Assert.equal example in issue #25. Locate the project entry point for the Size-related graphics code, add the function there, and run the project tests; done means the shown 4×3 image in a 3×4 viewport satisfies the asserted result.

Written by the indexing model from the issue text.