feat: add ExpressionEvaluator for G-code variable expressions

Also set ContinueOnError=true on Cincinnati's post-build copy to prevent
the running WinForms app from blocking test builds via a file lock.

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

OpenNest.Tests/Math/ExpressionEvaluatorTests.cs

@@ -0,0 +1,119 @@
+using System.Collections.Generic;
+using OpenNest.Math;
+
+namespace OpenNest.Tests.Math;
+
+public class ExpressionEvaluatorTests
+{
+    private static readonly Dictionary<string, double> Empty = new();
+
+    [Fact]
+    public void Evaluate_NumericLiteral()
+    {
+        Assert.Equal(42.0, ExpressionEvaluator.Evaluate("42", Empty));
+    }
+
+    [Fact]
+    public void Evaluate_DecimalLiteral()
+    {
+        Assert.Equal(0.3, ExpressionEvaluator.Evaluate("0.3", Empty));
+    }
+
+    [Fact]
+    public void Evaluate_NegativeLiteral()
+    {
+        Assert.Equal(-5.0, ExpressionEvaluator.Evaluate("-5", Empty));
+    }
+
+    [Fact]
+    public void Evaluate_Addition()
+    {
+        Assert.Equal(5.0, ExpressionEvaluator.Evaluate("2 + 3", Empty));
+    }
+
+    [Fact]
+    public void Evaluate_Subtraction()
+    {
+        Assert.Equal(1.0, ExpressionEvaluator.Evaluate("3 - 2", Empty));
+    }
+
+    [Fact]
+    public void Evaluate_Multiplication()
+    {
+        Assert.Equal(6.0, ExpressionEvaluator.Evaluate("2 * 3", Empty));
+    }
+
+    [Fact]
+    public void Evaluate_Division()
+    {
+        Assert.Equal(2.5, ExpressionEvaluator.Evaluate("5 / 2", Empty));
+    }
+
+    [Fact]
+    public void Evaluate_OperatorPrecedence()
+    {
+        Assert.Equal(7.0, ExpressionEvaluator.Evaluate("1 + 2 * 3", Empty));
+    }
+
+    [Fact]
+    public void Evaluate_Parentheses()
+    {
+        Assert.Equal(9.0, ExpressionEvaluator.Evaluate("(1 + 2) * 3", Empty));
+    }
+
+    [Fact]
+    public void Evaluate_VariableReference()
+    {
+        var vars = new Dictionary<string, double> { { "diameter", 0.3 } };
+        Assert.Equal(0.3, ExpressionEvaluator.Evaluate("$diameter", vars));
+    }
+
+    [Fact]
+    public void Evaluate_VariableInExpression()
+    {
+        var vars = new Dictionary<string, double> { { "diameter", 0.6 } };
+        Assert.Equal(0.3, ExpressionEvaluator.Evaluate("$diameter / 2", vars));
+    }
+
+    [Fact]
+    public void Evaluate_MultipleVariables()
+    {
+        var vars = new Dictionary<string, double> { { "x", 2.0 }, { "y", 3.0 } };
+        Assert.Equal(5.0, ExpressionEvaluator.Evaluate("$x + $y", vars));
+    }
+
+    [Fact]
+    public void Evaluate_CaseInsensitiveVariables()
+    {
+        var vars = new Dictionary<string, double>(StringComparer.OrdinalIgnoreCase)
+        {
+            { "Diameter", 0.3 }
+        };
+        Assert.Equal(0.3, ExpressionEvaluator.Evaluate("$diameter", vars));
+    }
+
+    [Fact]
+    public void Evaluate_UndefinedVariable_Throws()
+    {
+        Assert.Throws<KeyNotFoundException>(() =>
+            ExpressionEvaluator.Evaluate("$missing", Empty));
+    }
+
+    [Fact]
+    public void Evaluate_NestedParentheses()
+    {
+        Assert.Equal(20.0, ExpressionEvaluator.Evaluate("((2 + 3) * (1 + 1)) * 2", Empty));
+    }
+
+    [Fact]
+    public void Evaluate_UnaryMinusBeforeParens()
+    {
+        Assert.Equal(-6.0, ExpressionEvaluator.Evaluate("-(2 + 4)", Empty));
+    }
+
+    [Fact]
+    public void Evaluate_WhitespaceHandling()
+    {
+        Assert.Equal(5.0, ExpressionEvaluator.Evaluate("  2  +  3  ", Empty));
+    }
+}