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/check.go

@@ -298,7 +298,11 @@ func (mfa *CLIApp) checkManifestOperation(ctx *cli.Context) error {
 	log.Infof("checking manifest %s with base %s", manifestPath, basePath)
 
 	// Create checker
-	chk, err := mfer.NewChecker(manifestPath, basePath, mfa.Fs)
+	chk, err := mfer.NewChecker(&mfer.CheckerOptions{
+		ManifestPath: manifestPath,
+		BasePath:     basePath,
+		Fs:           mfa.Fs,
+	})
 	if err != nil {
 		return fmt.Errorf("failed to load manifest: %w", err)
 	}