feat(core): add TShape

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

OpenNest.Core/Shapes/TShape.cs

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+using System.Collections.Generic;
+using OpenNest.Geometry;
+
+namespace OpenNest.Shapes
+{
+    public class TShape : ShapeDefinition
+    {
+        public double Width { get; set; }
+        public double Height { get; set; }
+        public double StemWidth { get; set; }
+        public double BarHeight { get; set; }
+
+        public override Drawing GetDrawing()
+        {
+            var sw = StemWidth > 0 ? StemWidth : Width / 3.0;
+            var bh = BarHeight > 0 ? BarHeight : Height / 3.0;
+            var stemLeft = (Width - sw) / 2.0;
+            var stemRight = stemLeft + sw;
+            var stemTop = Height - bh;
+
+            var entities = new List<Entity>
+            {
+                new Line(stemLeft, 0, stemRight, 0),
+                new Line(stemRight, 0, stemRight, stemTop),
+                new Line(stemRight, stemTop, Width, stemTop),
+                new Line(Width, stemTop, Width, Height),
+                new Line(Width, Height, 0, Height),
+                new Line(0, Height, 0, stemTop),
+                new Line(0, stemTop, stemLeft, stemTop),
+                new Line(stemLeft, stemTop, stemLeft, 0)
+            };
+
+            return CreateDrawing(entities);
+        }
+    }
+}

OpenNest.Tests/Shapes/TShapeTests.cs

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+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);
+    }
+}