feat: add ProgramVariable and ProgramVariableManager for macro variable declarations

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

OpenNest.Posts.Cincinnati/ProgramVariable.cs

@@ -0,0 +1,18 @@
+namespace OpenNest.Posts.Cincinnati
+{
+    public sealed class ProgramVariable
+    {
+        public int Number { get; }
+        public string Name { get; }
+        public string Expression { get; set; }
+
+        public ProgramVariable(int number, string name, string expression = null)
+        {
+            Number = number;
+            Name = name;
+            Expression = expression;
+        }
+
+        public string Reference => $"#{Number}";
+    }
+}