NewChecker takes CheckerOptions instead of positional arguments (closes #78)

NewChecker now takes *CheckerOptions (ManifestPath, BasePath, Fs), named
like ScannerOptions. A nil Fs still means the OS filesystem, as before and
as in ScannerOptions; nil options or an empty path return an error naming
the missing path.

Audit of the other exported constructors in mfer: NewManifestFromFile took
a filesystem and a path positionally; it now takes
*ManifestFromFileOptions (Path, Fs) with the same nil and empty rules.
NewBuilder and NewScanner take no arguments, NewScannerWithOptions already
takes options, and NewManifestFromReader takes one reader, which the style
guide exempts; these are unchanged.

Model: opus-5-5

mfer/deserialize.go

@@ -352,16 +352,29 @@ func NewManifestFromReader(input io.Reader) (*manifest, error) {
 	return m, nil
 }
 
-// NewManifestFromFile reads a manifest from a file path using the given filesystem.
-// If fs is nil, the real filesystem (OsFs) is used.
+// ManifestFromFileOptions configures NewManifestFromFile.
+type ManifestFromFileOptions struct {
+	// Path is the manifest file to read (required).
+	Path string
+	// Fs is the filesystem to use, defaults to OsFs if nil.
+	Fs afero.Fs
+}
+
+// NewManifestFromFile reads a manifest from a file. It returns an error if
+// opts is nil or its path is empty.
 //
 //nolint:revive // unexported-return: exporting manifest is owner question 13
-func NewManifestFromFile(fs afero.Fs, path string) (*manifest, error) {
+func NewManifestFromFile(opts *ManifestFromFileOptions) (*manifest, error) {
+	if opts == nil || opts.Path == "" {
+		return nil, errManifestPathEmpty
+	}
+
+	fs := opts.Fs
 	if fs == nil {
 		fs = afero.NewOsFs()
 	}
 
-	f, err := fs.Open(path)
+	f, err := fs.Open(opts.Path)
 	if err != nil {
 		return nil, err
 	}