refactor: move ProgramVariableManager to Core, add config file support

- Move ProgramVariable and ProgramVariableManager from
  OpenNest.Posts.Cincinnati to OpenNest.Core/CNC (namespace OpenNest.CNC)
  so they can be used internally in nest programs
- Add parameterless constructor to CincinnatiPostProcessor that loads
  config from a .json file next to the DLL (creates defaults on first run)
- Enums serialize as readable strings (e.g., "Inches", "ControllerSide")
- Config file: OpenNest.Posts.Cincinnati.json in the Posts/ directory

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

OpenNest.Core/CNC/ProgramVariableManager.cs

@@ -0,0 +1,43 @@
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+
+namespace OpenNest.CNC
+{
+    public sealed class ProgramVariableManager
+    {
+        private readonly Dictionary<int, ProgramVariable> _variables = new();
+
+        public ProgramVariable GetOrCreate(string name, int number, string expression = null)
+        {
+            if (_variables.TryGetValue(number, out var existing))
+                return existing;
+
+            var variable = new ProgramVariable(number, name, expression);
+            _variables[number] = variable;
+            return variable;
+        }
+
+        public List<string> EmitDeclarations()
+        {
+            return _variables.Values
+                .Where(v => v.Expression != null)
+                .OrderBy(v => v.Number)
+                .Select(v => $"{v.Reference}={v.Expression} ({FormatComment(v.Name)})")
+                .ToList();
+        }
+
+        private static string FormatComment(string name)
+        {
+            // "LeadInFeedrate" -> "LEAD IN FEEDRATE"
+            var sb = new StringBuilder();
+            foreach (var c in name)
+            {
+                if (char.IsUpper(c) && sb.Length > 0)
+                    sb.Append(' ');
+                sb.Append(char.ToUpper(c));
+            }
+            return sb.ToString();
+        }
+    }
+}