Executable code cells keep their Run button on file-backed meshes

Code cells marked executable used to silently lose that setting — together with their execution history and activity links — whenever the mesh stored them on a file system, in blob storage, or synced them to a git repository. After a restart or re-import the Run button was gone and running the script was refused.

Such cells are now stored in a lossless format, so the Run button, the last-run information and the activity trail all survive persistence and sync. Plain C# source files are unaffected and keep their readable .cs form in synced repositories.