feat(ui): add DensityBar sparkline control for density visualization

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

OpenNest/Controls/DensityBar.cs

@@ -0,0 +1,70 @@
+using System.Drawing;
+using System.Drawing.Drawing2D;
+using System.Windows.Forms;
+
+namespace OpenNest.Controls
+{
+    public class DensityBar : Control
+    {
+        private static readonly Color TrackColor = Color.FromArgb(224, 224, 224);
+        private static readonly Color LowColor = Color.FromArgb(245, 166, 35);
+        private static readonly Color HighColor = Color.FromArgb(76, 175, 80);
+
+        private double value;
+
+        public DensityBar()
+        {
+            DoubleBuffered = true;
+            SetStyle(ControlStyles.OptimizedDoubleBuffer | ControlStyles.AllPaintingInWmPaint | ControlStyles.UserPaint, true);
+            Size = new Size(60, 8);
+        }
+
+        public double Value
+        {
+            get => value;
+            set
+            {
+                this.value = System.Math.Clamp(value, 0.0, 1.0);
+                Invalidate();
+            }
+        }
+
+        protected override void OnPaint(PaintEventArgs e)
+        {
+            base.OnPaint(e);
+            var g = e.Graphics;
+            g.SmoothingMode = SmoothingMode.AntiAlias;
+
+            var rect = new Rectangle(0, 0, Width - 1, Height - 1);
+
+            // Track background
+            using var trackPath = CreateRoundedRect(rect, 4);
+            using var trackBrush = new SolidBrush(TrackColor);
+            g.FillPath(trackBrush, trackPath);
+
+            // Fill
+            var fillWidth = (int)(rect.Width * value);
+            if (fillWidth > 0)
+            {
+                var fillRect = new Rectangle(rect.X, rect.Y, fillWidth, rect.Height);
+                using var fillPath = CreateRoundedRect(fillRect, 4);
+                using var gradientBrush = new LinearGradientBrush(
+                    new Point(rect.X, 0), new Point(rect.Right, 0),
+                    LowColor, HighColor);
+                g.FillPath(gradientBrush, fillPath);
+            }
+        }
+
+        private static GraphicsPath CreateRoundedRect(Rectangle rect, int radius)
+        {
+            var path = new GraphicsPath();
+            var d = radius * 2;
+            path.AddArc(rect.X, rect.Y, d, d, 180, 90);
+            path.AddArc(rect.Right - d, rect.Y, d, d, 270, 90);
+            path.AddArc(rect.Right - d, rect.Bottom - d, d, d, 0, 90);
+            path.AddArc(rect.X, rect.Bottom - d, d, d, 90, 90);
+            path.CloseFigure();
+            return path;
+        }
+    }
+}