fix: assign part colors to drawings created by BOM importer and MCP

Drawings created by BomImportForm and MCP InputTools were missing color
assignments, causing them to render with default empty color instead of
the standard part color palette. Moved PartColors and GetNextColor() to
Drawing in Core so all consumers share one definition.

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

OpenNest/ColorScheme.cs

@@ -13,21 +13,7 @@ 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 Color[] PartColors => Drawing.PartColors;
 
         public static readonly ColorScheme Default = new ColorScheme
         {