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.Training/Program.cs

@@ -79,21 +79,7 @@ int RunDataCollection(string dir, string dbPath, string saveDir, double s, strin
         Console.WriteLine($"Using template: {template}");
     }
 
-    var PartColors = new[]
-    {
-        Color.FromArgb(205, 92, 92),
-        Color.FromArgb(148, 103, 189),
-        Color.FromArgb(75, 180, 175),
-        Color.FromArgb(210, 190, 75),
-        Color.FromArgb(190, 85, 175),
-        Color.FromArgb(185, 115, 85),
-        Color.FromArgb(120, 100, 190),
-        Color.FromArgb(200, 100, 140),
-        Color.FromArgb(80, 175, 155),
-        Color.FromArgb(195, 160, 85),
-        Color.FromArgb(175, 95, 160),
-        Color.FromArgb(215, 130, 130),
-    };
+    var PartColors = Drawing.PartColors;
 
     var sheetSuite = new[]
     {