feat(engine): add IterativeShrinkFiller with dual-direction shrink selection

Introduces IterativeShrinkFiller.Fill, which composes RemnantFiller and
ShrinkFiller by wrapping the caller's fill function in a closure that tries
both ShrinkAxis.Height and ShrinkAxis.Width and picks the better FillScore.
Adds IterativeShrinkResult (Parts + Leftovers). Covers null/empty inputs and
single-item placement with three passing xUnit tests.

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

OpenNest.Tests/IterativeShrinkFillerTests.cs

@@ -0,0 +1,59 @@
+using OpenNest.Engine.Fill;
+using OpenNest.Geometry;
+
+namespace OpenNest.Tests;
+
+public class IterativeShrinkFillerTests
+{
+    [Fact]
+    public void Fill_NullItems_ReturnsEmpty()
+    {
+        Func<NestItem, Box, List<Part>> fillFunc = (ni, b) => new List<Part>();
+        var result = IterativeShrinkFiller.Fill(null, new Box(0, 0, 100, 100), fillFunc, 1.0);
+
+        Assert.Empty(result.Parts);
+        Assert.Empty(result.Leftovers);
+    }
+
+    [Fact]
+    public void Fill_EmptyItems_ReturnsEmpty()
+    {
+        Func<NestItem, Box, List<Part>> fillFunc = (ni, b) => new List<Part>();
+        var result = IterativeShrinkFiller.Fill(new List<NestItem>(), new Box(0, 0, 100, 100), fillFunc, 1.0);
+
+        Assert.Empty(result.Parts);
+        Assert.Empty(result.Leftovers);
+    }
+
+    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 Fill_SingleItem_PlacesParts()
+    {
+        var drawing = MakeRectDrawing(20, 10);
+        var items = new List<NestItem>
+        {
+            new NestItem { Drawing = drawing, Quantity = 5 }
+        };
+
+        Func<NestItem, Box, List<Part>> fillFunc = (ni, b) =>
+        {
+            var plate = new Plate(b.Width, b.Length);
+            var engine = new DefaultNestEngine(plate);
+            return engine.Fill(ni, b, null, System.Threading.CancellationToken.None);
+        };
+
+        var result = IterativeShrinkFiller.Fill(items, new Box(0, 0, 120, 60), fillFunc, 1.0);
+
+        Assert.True(result.Parts.Count > 0, "Should place parts");
+    }
+}