tuist generate leaves project.pbxproj untouched when output is unchanged

Running tuist generate on an unchanged input no longer rewrites project.pbxproj. Tuist serializes the project once, compares to the file on disk, and skips the write when contents match. The file’s modification time and inode are preserved, so no-op regenerations no longer show up as spurious git changes or trigger file-watcher wakeups. Real structural regenerations behave exactly as before.