feat: add Plate.GetRemnants() for finding empty edge strips

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

OpenNest.Core/Plate.cs

@@ -473,5 +473,87 @@ namespace OpenNest
 
             return pts.Count > 0;
         }
+
+        /// <summary>
+        /// Finds rectangular remnant (empty) regions on the plate.
+        /// Returns strips along edges that are clear of parts.
+        /// </summary>
+        public List<Box> GetRemnants()
+        {
+            var work = WorkArea();
+            var results = new List<Box>();
+
+            if (Parts.Count == 0)
+            {
+                results.Add(work);
+                return results;
+            }
+
+            var obstacles = new List<Box>();
+            foreach (var part in Parts)
+                obstacles.Add(part.BoundingBox.Offset(PartSpacing));
+
+            // Right strip: from the rightmost part edge to the work area right edge
+            var maxRight = double.MinValue;
+            foreach (var box in obstacles)
+            {
+                if (box.Right > maxRight)
+                    maxRight = box.Right;
+            }
+
+            if (maxRight < work.Right)
+            {
+                var strip = new Box(maxRight, work.Bottom, work.Right - maxRight, work.Height);
+                if (strip.Area() > 1.0)
+                    results.Add(strip);
+            }
+
+            // Top strip: from the topmost part edge to the work area top edge
+            var maxTop = double.MinValue;
+            foreach (var box in obstacles)
+            {
+                if (box.Top > maxTop)
+                    maxTop = box.Top;
+            }
+
+            if (maxTop < work.Top)
+            {
+                var strip = new Box(work.Left, maxTop, work.Width, work.Top - maxTop);
+                if (strip.Area() > 1.0)
+                    results.Add(strip);
+            }
+
+            // Bottom strip: from work area bottom to the lowest part edge
+            var minBottom = double.MaxValue;
+            foreach (var box in obstacles)
+            {
+                if (box.Bottom < minBottom)
+                    minBottom = box.Bottom;
+            }
+
+            if (minBottom > work.Bottom)
+            {
+                var strip = new Box(work.Left, work.Bottom, work.Width, minBottom - work.Bottom);
+                if (strip.Area() > 1.0)
+                    results.Add(strip);
+            }
+
+            // Left strip: from work area left to the leftmost part edge
+            var minLeft = double.MaxValue;
+            foreach (var box in obstacles)
+            {
+                if (box.Left < minLeft)
+                    minLeft = box.Left;
+            }
+
+            if (minLeft > work.Left)
+            {
+                var strip = new Box(work.Left, work.Bottom, minLeft - work.Left, work.Height);
+                if (strip.Area() > 1.0)
+                    results.Add(strip);
+            }
+
+            return results;
+        }
     }
 }