fix(ui): handle edge cases in DensityBar and PhaseStepperControl

DensityBar: clamp rounded rect radius for small fill widths to avoid
GDI+ artifacts at very low density values.

PhaseStepperControl: use float arithmetic for circle spacing to
handle DPI-scaled widths evenly.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

OpenNest/Controls/DensityBar.cs

@@ -46,8 +46,9 @@ namespace OpenNest.Controls
             var fillWidth = (int)(rect.Width * value);
             if (fillWidth > 0)
             {
+                var fillRadius = System.Math.Min(4, fillWidth / 2);
                 var fillRect = new Rectangle(rect.X, rect.Y, fillWidth, rect.Height);
-                using var fillPath = CreateRoundedRect(fillRect, 4);
+                using var fillPath = CreateRoundedRect(fillRect, fillRadius);
                 using var gradientBrush = new LinearGradientBrush(
                     new Point(rect.X, 0), new Point(rect.Right, 0),
                     LowColor, HighColor);

OpenNest/Controls/PhaseStepperControl.cs

@@ -81,7 +81,7 @@ namespace OpenNest.Controls
 
             var padding = 30;
             var usableWidth = Width - padding * 2;
-            var spacing = usableWidth / (count - 1);
+            var spacing = (float)usableWidth / (count - 1);
             var circleY = 18;
             var normalRadius = 9;
             var activeRadius = 11;
@@ -96,8 +96,8 @@ namespace OpenNest.Controls
             // Draw connecting lines
             for (var i = 0; i < count - 1; i++)
             {
-                var x1 = padding + i * spacing;
+                var x1 = (int)(padding + i * spacing);
-                var x2 = padding + (i + 1) * spacing;
+                var x2 = (int)(padding + (i + 1) * spacing);
                 g.DrawLine(linePen, x1, circleY, x2, circleY);
             }
 
@@ -105,7 +105,7 @@ namespace OpenNest.Controls
             for (var i = 0; i < count; i++)
             {
                 var phase = Phases[i];
-                var cx = padding + i * spacing;
+                var cx = (int)(padding + i * spacing);
                 var isActive = activePhase == phase && !isComplete;
                 var isVisited = visitedPhases.Contains(phase) || isComplete;