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.Mcp/Tools/InputTools.cs

@@ -112,6 +112,7 @@ namespace OpenNest.Mcp.Tools
 
             var drawingName = name ?? Path.GetFileNameWithoutExtension(path);
             var drawing = new Drawing(drawingName, pgm);
+            drawing.Color = Drawing.GetNextColor();
             _session.Drawings.Add(drawing);
 
             var bbox = pgm.BoundingBox();
@@ -155,6 +156,7 @@ namespace OpenNest.Mcp.Tools
                     if (pgm == null)
                         return "Error: failed to parse G-code";
                     var gcodeDrawing = new Drawing(name, pgm);
+                    gcodeDrawing.Color = Drawing.GetNextColor();
                     _session.Drawings.Add(gcodeDrawing);
                     var gcodeBbox = pgm.BoundingBox();
                     return $"Created drawing '{name}': bbox={gcodeBbox.Width:F2} x {gcodeBbox.Length:F2}";
@@ -164,6 +166,7 @@ namespace OpenNest.Mcp.Tools
             }
 
             var drawing = shapeDef.GetDrawing();
+            drawing.Color = Drawing.GetNextColor();
             _session.Drawings.Add(drawing);
 
             var bbox = drawing.Program.BoundingBox();