- `Project::to_file_json` writes `savedBy` (the EffectCraft version) ahead of the project and
checks that the text reads back before anything is written: JSON has no NaN or infinity, so
such a value made a file that saved but could never be opened. Save now fails with a message
and the file on disk is left as it was.
- Opening a project saved by a newer EffectCraft warns that settings this version doesn't know
are lost on saving; `file.open` reports `savedBy`. A file that isn't a project names itself in
the error.
- Increment and Save keeps the extension (`.ecprojx` stays XML); Save a Copy writes XML for
`.ecprojx` / `.xml` paths, like Save As.
- Docs: architecture (document model), parity.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>