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/builder_test.go

@@ -5,10 +5,12 @@ import (
 	"bytes"
 	"context"
 	"fmt"
+	"path/filepath"
 	"strings"
 	"testing"
 	"time"
 
+	"github.com/spf13/afero"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
@@ -397,6 +399,29 @@ func TestNewManifestFromReaderTruncated(t *testing.T) {
 	assert.Error(t, err)
 }
 
+func TestNewManifestFromFileRequiresPath(t *testing.T) {
+	t.Parallel()
+
+	_, err := NewManifestFromFile(nil)
+	require.ErrorIs(t, err, errManifestPathEmpty)
+
+	_, err = NewManifestFromFile(&ManifestFromFileOptions{Fs: afero.NewMemMapFs()})
+	require.ErrorIs(t, err, errManifestPathEmpty)
+}
+
+func TestNewManifestFromFileNilFsUsesOsFs(t *testing.T) {
+	t.Parallel()
+
+	path := filepath.Join(t.TempDir(), "index.mf")
+	createTestManifest(t, afero.NewOsFs(), path, map[string][]byte{
+		testFileName: []byte("hello"),
+	})
+
+	m, err := NewManifestFromFile(&ManifestFromFileOptions{Path: path})
+	require.NoError(t, err)
+	assert.Len(t, m.Files(), 1)
+}
+
 func TestManifestString(t *testing.T) {
 	t.Parallel()