fix(desktop): paint the utilization bar when its fill is a sliver

A one-pixel fill (about 2% on the Nesting Progress bar) asked GDI+ for a
zero-size corner arc, which threw "Parameter is not valid" mid-nest. Clamp
the corner radius to the rectangle and fall back to a plain rectangle.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

OpenNest.WinForms.Tests/Controls/DensityBarTests.cs

@@ -0,0 +1,33 @@
+using System.Drawing;
+using System.Reflection;
+using System.Windows.Forms;
+using OpenNest.Controls;
+
+namespace OpenNest.WinForms.Tests.Controls;
+
+/// <summary>
+/// The Nesting Progress utilization bar paints at every value and size: a sliver of fill
+/// (about 1% on the default bar) once threw from a zero-size corner arc.
+/// </summary>
+public class DensityBarTests
+{
+    [Theory]
+    [InlineData(60, 8, 0.0)]
+    [InlineData(60, 8, 0.01)]
+    [InlineData(60, 8, 0.02)]
+    [InlineData(60, 8, 0.05)]
+    [InlineData(60, 8, 0.5)]
+    [InlineData(60, 8, 1.0)]
+    [InlineData(60, 4, 0.5)]
+    [InlineData(3, 2, 1.0)]
+    [InlineData(1, 1, 1.0)]
+    public void Paints_without_throwing(int width, int height, double value)
+    {
+        using var bar = new DensityBar { Size = new Size(width, height), Value = value };
+        using var bitmap = new Bitmap(System.Math.Max(width, 1), System.Math.Max(height, 1));
+        using var g = Graphics.FromImage(bitmap);
+
+        typeof(DensityBar).GetMethod("OnPaint", BindingFlags.Instance | BindingFlags.NonPublic)!
+            .Invoke(bar, new object[] { new PaintEventArgs(g, bar.ClientRectangle) });
+    }
+}

OpenNest/Controls/DensityBar.cs

@@ -67,6 +67,14 @@ namespace OpenNest.Controls
         private static GraphicsPath CreateRoundedRect(Rectangle rect, int radius)
         {
             var path = new GraphicsPath();
+            // GDI+ refuses a zero-size arc, which a sliver of fill would need.
+            radius = System.Math.Min(radius, System.Math.Min(rect.Width, rect.Height) / 2);
+            if (radius <= 0)
+            {
+                path.AddRectangle(rect);
+                return path;
+            }
+
             var d = radius * 2;
             path.AddArc(rect.X, rect.Y, d, d, 180, 90);
             path.AddArc(rect.Right - d, rect.Y, d, d, 270, 90);