feat: add PlateOptimizer with cost-aware plate size selection

Tries each candidate plate size via the nesting engine, compares results
by part count then net cost (accounting for salvage credit on remnant
material), and returns the best option.

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

OpenNest.Tests/Engine/PlateOptimizerTests.cs

@@ -0,0 +1,127 @@
+using OpenNest.Geometry;
+
+namespace OpenNest.Tests.Engine;
+
+public class PlateOptimizerTests
+{
+    private static Drawing MakeRectDrawing(double w, double h, string name = "rect")
+    {
+        var pgm = new OpenNest.CNC.Program();
+        pgm.Codes.Add(new OpenNest.CNC.RapidMove(new Vector(0, 0)));
+        pgm.Codes.Add(new OpenNest.CNC.LinearMove(new Vector(w, 0)));
+        pgm.Codes.Add(new OpenNest.CNC.LinearMove(new Vector(w, h)));
+        pgm.Codes.Add(new OpenNest.CNC.LinearMove(new Vector(0, h)));
+        pgm.Codes.Add(new OpenNest.CNC.LinearMove(new Vector(0, 0)));
+        return new Drawing(name, pgm);
+    }
+
+    [Fact]
+    public void PicksCheapestPlateThatFitsParts()
+    {
+        var options = new List<PlateOption>
+        {
+            new() { Width = 20, Length = 20, Cost = 100 },
+            new() { Width = 40, Length = 40, Cost = 400 },
+        };
+
+        var templatePlate = new Plate(40, 40) { PartSpacing = 0 };
+        var items = new List<NestItem>
+        {
+            new() { Drawing = MakeRectDrawing(10, 10), Quantity = 1 }
+        };
+
+        var result = PlateOptimizer.Optimize(items, options, 0.0, templatePlate);
+
+        Assert.NotNull(result);
+        Assert.Equal(20, result.ChosenSize.Width);
+        Assert.True(result.Parts.Count >= 1);
+    }
+
+    [Fact]
+    public void PrefersMorePartsOverCheaperPlate()
+    {
+        var options = new List<PlateOption>
+        {
+            new() { Width = 12, Length = 12, Cost = 50 },
+            new() { Width = 24, Length = 12, Cost = 100 },
+        };
+
+        var templatePlate = new Plate(24, 12) { PartSpacing = 0 };
+        var items = new List<NestItem>
+        {
+            new() { Drawing = MakeRectDrawing(10, 10), Quantity = 2 }
+        };
+
+        var result = PlateOptimizer.Optimize(items, options, 0.0, templatePlate);
+
+        Assert.NotNull(result);
+        Assert.Equal(24, result.ChosenSize.Width);
+        Assert.Equal(2, result.Parts.Count);
+    }
+
+    [Fact]
+    public void SalvageRateReducesNetCost()
+    {
+        // Small: 20x20=400sqin, cost $400. Part=10x10=100sqin. Remnant=300.
+        //   Net = 400 - 300*(400/400)*1.0 = 400-300 = 100
+        // Large: 40x40=1600sqin, cost $800. Part=10x10=100sqin. Remnant=1500.
+        //   Net = 800 - 1500*(800/1600)*1.0 = 800-750 = 50
+        var options = new List<PlateOption>
+        {
+            new() { Width = 20, Length = 20, Cost = 400 },
+            new() { Width = 40, Length = 40, Cost = 800 },
+        };
+
+        var templatePlate = new Plate(40, 40) { PartSpacing = 0 };
+        templatePlate.EdgeSpacing = new Spacing();
+        var items = new List<NestItem>
+        {
+            new() { Drawing = MakeRectDrawing(10, 10), Quantity = 1 }
+        };
+
+        var result = PlateOptimizer.Optimize(items, options, 1.0, templatePlate);
+
+        Assert.NotNull(result);
+        Assert.Equal(40, result.ChosenSize.Width);
+    }
+
+    [Fact]
+    public void SkipsPlatesThatAreTooSmall()
+    {
+        var options = new List<PlateOption>
+        {
+            new() { Width = 20, Length = 20, Cost = 100 },
+            new() { Width = 40, Length = 40, Cost = 400 },
+        };
+
+        var templatePlate = new Plate(40, 40) { PartSpacing = 0 };
+        var items = new List<NestItem>
+        {
+            new() { Drawing = MakeRectDrawing(30, 30), Quantity = 1 }
+        };
+
+        var result = PlateOptimizer.Optimize(items, options, 0.0, templatePlate);
+
+        Assert.NotNull(result);
+        Assert.Equal(40, result.ChosenSize.Width);
+    }
+
+    [Fact]
+    public void ReturnsNullWhenNoPlatesFit()
+    {
+        var options = new List<PlateOption>
+        {
+            new() { Width = 10, Length = 10, Cost = 50 },
+        };
+
+        var templatePlate = new Plate(10, 10) { PartSpacing = 0 };
+        var items = new List<NestItem>
+        {
+            new() { Drawing = MakeRectDrawing(20, 20), Quantity = 1 }
+        };
+
+        var result = PlateOptimizer.Optimize(items, options, 0.0, templatePlate);
+
+        Assert.Null(result);
+    }
+}