feat(core): add TShape

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

OpenNest.Tests/Shapes/TShapeTests.cs

@@ -0,0 +1,40 @@
+using OpenNest.Shapes;
+
+namespace OpenNest.Tests.Shapes;
+
+public class TShapeTests
+{
+    [Fact]
+    public void GetDrawing_BoundingBoxMatchesDimensions()
+    {
+        var shape = new TShape { Width = 12, Height = 18 };
+        var drawing = shape.GetDrawing();
+
+        var bbox = drawing.Program.BoundingBox();
+        Assert.Equal(12, bbox.Width, 0.01);
+        Assert.Equal(18, bbox.Length, 0.01);
+    }
+
+    [Fact]
+    public void GetDrawing_DefaultStemAndBarDimensions()
+    {
+        var shape = new TShape { Width = 12, Height = 18 };
+        var drawing = shape.GetDrawing();
+
+        // Default: StemWidth = Width/3 = 4, BarHeight = Height/3 = 6
+        // Area = Width * BarHeight + StemWidth * (Height - BarHeight)
+        // Area = 12 * 6 + 4 * 12 = 72 + 48 = 120
+        Assert.Equal(120, drawing.Area, 0.5);
+    }
+
+    [Fact]
+    public void GetDrawing_CustomStemAndBarDimensions()
+    {
+        var shape = new TShape { Width = 12, Height = 18, StemWidth = 6, BarHeight = 4 };
+        var drawing = shape.GetDrawing();
+
+        // Area = Width * BarHeight + StemWidth * (Height - BarHeight)
+        // Area = 12 * 4 + 6 * 14 = 48 + 84 = 132
+        Assert.Equal(132, drawing.Area, 0.5);
+    }
+}