pub fn migrate_to_sidecars(data_dir: &Path) -> Result<Migration>Expand description
manifest migrate-to-sidecars: appends each value the current manifest
carries and its package’s sidecar lacks, so a pure regeneration keeps it.
Never overwrites a key, never pins the legacy or a derived version, and a second run writes nothing; appending keeps a sidecar’s comments, and a top-level key is valid at the end of a flat TOML file.