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

internal/cli/freshen.go

@@ -445,7 +445,10 @@ func (mfa *CLIApp) loadExistingEntries(
 	log.Infof("loading manifest from %s", manifestPath)
 
 	// Load existing manifest
-	manifest, err := mfer.NewManifestFromFile(mfa.Fs, manifestPath)
+	manifest, err := mfer.NewManifestFromFile(&mfer.ManifestFromFileOptions{
+		Path: manifestPath,
+		Fs:   mfa.Fs,
+	})
 	if err != nil {
 		return nil, fmt.Errorf("failed to load manifest: %w", err)
 	}