refactor(desktop): share the separator line controls' pens and painting

HorizontalLine and VerticalLine had identical constructors, Dispose and
resize handling. They now derive from SeparatorLine and only say which
way the line runs. No behavior change.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

OpenNest.WinForms.Tests/Controls/SeparatorPenLifetimeTests.cs

@@ -75,7 +75,7 @@ public class SeparatorPenLifetimeTests
         horizontal ? new HorizontalLine() : new VerticalLine();
 
     private static Pen GetPen(Control control, string name) =>
-        Assert.IsType<Pen>(control.GetType()
+        Assert.IsType<Pen>(typeof(SeparatorLine)
             .GetField(name, BindingFlags.Instance | BindingFlags.NonPublic)!.GetValue(control));
 
     private static bool GetStyle(Control control, ControlStyles style) =>

OpenNest/Controls/HorizontalLine.cs

@@ -1,54 +1,12 @@
-using System;
 using System.Drawing;
-using System.Windows.Forms;
 
 namespace OpenNest.Controls
 {
-    public class HorizontalLine : Control
+    public class HorizontalLine : SeparatorLine
     {
-        private readonly Pen lightPen;
-        private readonly Pen darkPen;
+        protected override int Across => Height;
 
-        public HorizontalLine()
-        {
-            SetStyle(ControlStyles.OptimizedDoubleBuffer, true);
-            SetStyle(ControlStyles.Selectable, false);
-
-            lightPen = new Pen(ProfessionalColors.SeparatorLight);
-            darkPen = new Pen(ProfessionalColors.SeparatorDark);
-        }
-
-        protected override void Dispose(bool disposing)
-        {
-            try
-            {
-                if (disposing)
-                {
-                    lightPen.Dispose();
-                    darkPen.Dispose();
-                }
-            }
-            finally
-            {
-                base.Dispose(disposing);
-            }
-        }
-
-        protected override void OnResize(EventArgs e)
-        {
-            base.OnResize(e);
-            Invalidate();
-        }
-
-        protected override void OnPaint(PaintEventArgs e)
-        {
-            base.OnPaint(e);
-
-            float midpoint = Height * 0.5f;
-
-            e.Graphics.DrawLine(darkPen, 0, midpoint, Width, midpoint);
-            midpoint++;
-            e.Graphics.DrawLine(lightPen, 0, midpoint, Width, midpoint);
-        }
+        protected override void DrawLine(Graphics graphics, Pen pen, float position) =>
+            graphics.DrawLine(pen, 0, position, Width, position);
     }
 }

OpenNest/Controls/SeparatorLine.cs

@@ -0,0 +1,61 @@
+using System;
+using System.Drawing;
+using System.Windows.Forms;
+
+namespace OpenNest.Controls
+{
+    /// <summary>An etched separator: a dark line at the middle with a light line beside it.</summary>
+    public abstract class SeparatorLine : Control
+    {
+        private readonly Pen lightPen;
+        private readonly Pen darkPen;
+
+        protected SeparatorLine()
+        {
+            SetStyle(ControlStyles.OptimizedDoubleBuffer, true);
+            SetStyle(ControlStyles.Selectable, false);
+
+            lightPen = new Pen(ProfessionalColors.SeparatorLight);
+            darkPen = new Pen(ProfessionalColors.SeparatorDark);
+        }
+
+        /// <summary>The control's size across the line: Height for a horizontal line.</summary>
+        protected abstract int Across { get; }
+
+        /// <summary>Draws the full-length line at <paramref name="position"/> across it.</summary>
+        protected abstract void DrawLine(Graphics graphics, Pen pen, float position);
+
+        protected override void Dispose(bool disposing)
+        {
+            try
+            {
+                if (disposing)
+                {
+                    lightPen.Dispose();
+                    darkPen.Dispose();
+                }
+            }
+            finally
+            {
+                base.Dispose(disposing);
+            }
+        }
+
+        protected override void OnResize(EventArgs e)
+        {
+            base.OnResize(e);
+            Invalidate();
+        }
+
+        protected override void OnPaint(PaintEventArgs e)
+        {
+            base.OnPaint(e);
+
+            float midpoint = Across * 0.5f;
+
+            DrawLine(e.Graphics, darkPen, midpoint);
+            midpoint++;
+            DrawLine(e.Graphics, lightPen, midpoint);
+        }
+    }
+}

OpenNest/Controls/VerticalLine.cs

@@ -1,54 +1,12 @@
-using System;
 using System.Drawing;
-using System.Windows.Forms;
 
 namespace OpenNest.Controls
 {
-    public class VerticalLine : Control
+    public class VerticalLine : SeparatorLine
     {
-        private readonly Pen lightPen;
-        private readonly Pen darkPen;
+        protected override int Across => Width;
 
-        public VerticalLine()
-        {
-            SetStyle(ControlStyles.OptimizedDoubleBuffer, true);
-            SetStyle(ControlStyles.Selectable, false);
-
-            lightPen = new Pen(ProfessionalColors.SeparatorLight);
-            darkPen = new Pen(ProfessionalColors.SeparatorDark);
-        }
-
-        protected override void Dispose(bool disposing)
-        {
-            try
-            {
-                if (disposing)
-                {
-                    lightPen.Dispose();
-                    darkPen.Dispose();
-                }
-            }
-            finally
-            {
-                base.Dispose(disposing);
-            }
-        }
-
-        protected override void OnResize(EventArgs e)
-        {
-            base.OnResize(e);
-            Invalidate();
-        }
-
-        protected override void OnPaint(PaintEventArgs e)
-        {
-            base.OnPaint(e);
-
-            float midpoint = Width * 0.5f;
-
-            e.Graphics.DrawLine(darkPen, midpoint, 0, midpoint, Height);
-            midpoint++;
-            e.Graphics.DrawLine(lightPen, midpoint, 0, midpoint, Height);
-        }
+        protected override void DrawLine(Graphics graphics, Pen pen, float position) =>
+            graphics.DrawLine(pen, position, 0, position, Height);
     }
 }