feat(ui): replace random part colors with curated palette

Avoids visual confusion with reserved UI colors (orange preview parts,
green/blue selection windows) by using a fixed 12-color palette that
skips those hue zones. Removes unused HSLColor and RandomColorGenerator.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

OpenNest/ColorScheme.cs

@@ -13,6 +13,22 @@ namespace OpenNest
         private Color edgeSpacingColor;
         private Color previewPartColor;
 
+        public static readonly Color[] PartColors = new Color[]
+        {
+            Color.FromArgb(205, 92, 92),    // Indian Red
+            Color.FromArgb(148, 103, 189),  // Medium Purple
+            Color.FromArgb(75, 180, 175),   // Teal
+            Color.FromArgb(210, 190, 75),   // Goldenrod
+            Color.FromArgb(190, 85, 175),   // Orchid
+            Color.FromArgb(185, 115, 85),   // Sienna
+            Color.FromArgb(120, 100, 190),  // Slate Blue
+            Color.FromArgb(200, 100, 140),  // Rose
+            Color.FromArgb(80, 175, 155),   // Sea Green
+            Color.FromArgb(195, 160, 85),   // Dark Khaki
+            Color.FromArgb(175, 95, 160),   // Plum
+            Color.FromArgb(215, 130, 130),  // Light Coral
+        };
+
         public static readonly ColorScheme Default = new ColorScheme
         {
             BackgroundColor = Color.DarkGray,