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.Tests/Cincinnati/ProgramVariableManagerTests.cs

@@ -1,4 +1,4 @@
-using OpenNest.Posts.Cincinnati;
+using OpenNest.CNC;
 
 namespace OpenNest.Tests.Cincinnati;