Implement IDisposable on VirtualizationInstance to prevent zombie processes

[miniksa]

VirtualizationInstance holds native ProjFS handles (PRJ_NAMESPACE_VIRTUALIZATION_CONTEXT) and GCHandles that must be released when the instance is no longer needed. Previously, StopVirtualizing() had to be called explicitly — if the process crashed or exited without calling it, ProjFS kept the virtualization root alive, creating zombie processes.

Changes:

• IVirtualizationInstance now extends IDisposable
• VirtualizationInstance implements full dispose pattern:
  • Dispose() calls PrjStopVirtualizing, frees GCHandles, frees Marshal.StringToHGlobalUni notification strings
  • Finalizer ~VirtualizationInstance() as safety net
  • StopVirtualizing() delegates to Dispose(true) for compat
  • Thread-safe: _disposed flag prevents double-free
• Fixed memory leak: _notificationRootStrings (allocated via Marshal.StringToHGlobalUni) and _notificationMappingsHandle were never freed in StopVirtualizing

Consumers can now use 'using' or 'using var' for automatic cleanup:
using var instance = new VirtualizationInstance(...);

Version bumped to 2.1.0 (breaking: IVirtualizationInstance now requires Dispose)