Don't delete files that aren't changing

The IDE experience, such as in VSCode, when codegen deletes a whole bunch of files and then regenerates then w/o change is annoying. You may see a ton of stuff in Explorer flash red. It would be nicer if codegen recognized it produced exactly the same output and didn't overwrite the existing file.

Start by locating the Rust codegen entry point that deletes and regenerates files, then reproduce the behavior with unchanged generated output. The work is done when codegen leaves existing files untouched when their content is identical, and the VSCode Explorer no longer shows unnecessary deletion and recreation activity.

Written by the indexing model from the issue text.