.NET Framework 4.0 API compatibility gate

This compile-only project links the production CommandKernel, AgentBridge, CLI, SQL trace, and workflow gateway sources against the official .NET Framework 4.0 reference assemblies. It prevents the regular net6 test harness from hiding accidental calls to APIs introduced after .NET Framework 4.0.

It does not replace the Windows build of the legacy WinForms/DevExpress solution. The customer acceptance build must still run Build-LegacyErpAcceptance.ps1 on Windows and verify the final PE, CLR, dependency, signature, and source-provenance contracts.