feat: add VariableDefinition type for G-code user variables

Adds immutable VariableDefinition record to OpenNest.CNC with name,
expression, resolved value, inline, and global flags. Fixes namespace
collision in PatternTilerTests and PolygonHelperTests caused by the new
OpenNest.Tests.CNC namespace.

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

OpenNest.Core/CNC/VariableDefinition.cs

@@ -0,0 +1,21 @@
+namespace OpenNest.CNC
+{
+    public sealed class VariableDefinition
+    {
+        public string Name { get; }
+        public string Expression { get; }
+        public double Value { get; }
+        public bool Inline { get; }
+        public bool Global { get; }
+
+        public VariableDefinition(string name, string expression, double value,
+            bool inline = false, bool global = false)
+        {
+            Name = name;
+            Expression = expression;
+            Value = value;
+            Inline = inline;
+            Global = global;
+        }
+    }
+}