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_test.go

@@ -58,7 +58,10 @@ func TestFreshenUnchanged(t *testing.T) {
 	setupFreshenDir(t, fs)
 
 	// Parse manifest to verify
-	manifest, err := mfer.NewManifestFromFile(fs, "/testdir/.index.mf")
+	manifest, err := mfer.NewManifestFromFile(&mfer.ManifestFromFileOptions{
+		Path: "/testdir/.index.mf",
+		Fs:   fs,
+	})
 	require.NoError(t, err)
 	assert.Len(t, manifest.Files(), 2)
 }
@@ -70,7 +73,10 @@ func TestFreshenWithChanges(t *testing.T) {
 	setupFreshenDir(t, fs)
 
 	// Verify initial manifest has 2 files
-	manifest, err := mfer.NewManifestFromFile(fs, "/testdir/.index.mf")
+	manifest, err := mfer.NewManifestFromFile(&mfer.ManifestFromFileOptions{
+		Path: "/testdir/.index.mf",
+		Fs:   fs,
+	})
 	require.NoError(t, err)
 	assert.Len(t, manifest.Files(), 2)