feat: add syntax highlighting to gcode editor

Switch gcodeEditor from TextBox to RichTextBox and colorize G-code
tokens: rapids (amber), linear cuts (green), arcs (blue), comments
(dim gray), and mode codes (purple).

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

OpenNest/Controls/ProgramEditorControl.Designer.cs

@@ -28,7 +28,7 @@ namespace OpenNest.Controls
             rightSplit = new System.Windows.Forms.SplitContainer();
             preview = new EntityView();
             editorPanel = new System.Windows.Forms.Panel();
-            gcodeEditor = new System.Windows.Forms.TextBox();
+            gcodeEditor = new System.Windows.Forms.RichTextBox();
             editorToolbar = new System.Windows.Forms.Panel();
             lblGcode = new System.Windows.Forms.Label();
             ((System.ComponentModel.ISupportInitialize)mainSplit).BeginInit();
@@ -185,10 +185,9 @@ namespace OpenNest.Controls
             gcodeEditor.Font = new System.Drawing.Font("Consolas", 10F);
             gcodeEditor.ForeColor = System.Drawing.Color.FromArgb(180, 200, 180);
             gcodeEditor.Location = new System.Drawing.Point(0, 30);
-            gcodeEditor.Multiline = true;
             gcodeEditor.Name = "gcodeEditor";
-            gcodeEditor.ScrollBars = System.Windows.Forms.ScrollBars.Both;
             gcodeEditor.ReadOnly = true;
+            gcodeEditor.ScrollBars = System.Windows.Forms.RichTextBoxScrollBars.Both;
             gcodeEditor.Size = new System.Drawing.Size(260, 470);
             gcodeEditor.TabIndex = 1;
             gcodeEditor.WordWrap = false;
@@ -251,7 +250,7 @@ namespace OpenNest.Controls
         private System.Windows.Forms.Panel editorPanel;
         private System.Windows.Forms.Panel editorToolbar;
         private System.Windows.Forms.Label lblGcode;
-        private System.Windows.Forms.TextBox gcodeEditor;
+        private System.Windows.Forms.RichTextBox gcodeEditor;
         private System.Windows.Forms.ContextMenuStrip contourMenu;
         private System.Windows.Forms.ToolStripMenuItem menuReverse;
         private System.Windows.Forms.ToolStripMenuItem menuMoveUp;

OpenNest/Controls/ProgramEditorControl.cs

@@ -5,6 +5,7 @@ using System;
 using System.Collections.Generic;
 using System.Drawing;
 using System.Linq;
+using System.Text.RegularExpressions;
 using System.Windows.Forms;
 
 namespace OpenNest.Controls
@@ -90,7 +91,9 @@ namespace OpenNest.Controls
 
         private void UpdateGcodeText()
         {
-            gcodeEditor.Text = Program != null ? FormatProgram(Program, contours) : string.Empty;
+            var text = Program != null ? FormatProgram(Program, contours) : string.Empty;
+            gcodeEditor.Text = text;
+            ApplyHighlighting();
         }
 
         private static string FormatProgram(Program pgm, List<ContourInfo> contours)
@@ -142,6 +145,45 @@ namespace OpenNest.Controls
             return System.Math.Round(value, 4).ToString("0.####", System.Globalization.CultureInfo.InvariantCulture);
         }
 
+        private void ApplyHighlighting()
+        {
+            var text = gcodeEditor.Text;
+            if (string.IsNullOrEmpty(text)) return;
+
+            gcodeEditor.SuspendLayout();
+
+            var rapidColor = Color.FromArgb(230, 180, 80);
+            var linearColor = Color.FromArgb(130, 200, 140);
+            var arcColor = Color.FromArgb(120, 160, 255);
+            var commentColor = Color.FromArgb(120, 120, 140);
+            var modeColor = Color.FromArgb(200, 140, 220);
+            var coordColor = Color.FromArgb(180, 200, 180);
+
+            gcodeEditor.SelectAll();
+            gcodeEditor.SelectionColor = coordColor;
+
+            var rules = new (Regex pattern, Color color)[]
+            {
+                (new Regex(@"^;.*$", RegexOptions.Multiline), commentColor),
+                (new Regex(@"^G9[01]\b", RegexOptions.Multiline), modeColor),
+                (new Regex(@"^G00\b", RegexOptions.Multiline), rapidColor),
+                (new Regex(@"^G01\b", RegexOptions.Multiline), linearColor),
+                (new Regex(@"^G0[23]\b", RegexOptions.Multiline), arcColor),
+            };
+
+            foreach (var (pattern, color) in rules)
+            {
+                foreach (Match match in pattern.Matches(text))
+                {
+                    gcodeEditor.Select(match.Index, match.Length);
+                    gcodeEditor.SelectionColor = color;
+                }
+            }
+
+            gcodeEditor.Select(0, 0);
+            gcodeEditor.ResumeLayout();
+        }
+
         private void RefreshPreview()
         {
             preview.ClearPenCache();