Function validate_store_layout 

pub fn validate_store_layout(
    data_dir: &Path,
) -> Result<Option<StorageEngine>, PortError>

Applies the existing-layout gate without creating or opening anything.

Diagnostics use this exact gate so they cannot call a store healthy when the next real kernel operation will refuse it. None means genuinely fresh: no stamp and no engine file. A stamp without a store file is also valid — startup may have stopped between stamping and first engine open.