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/entry_test.go

@@ -504,7 +504,10 @@ func TestGenerateExcludesDotfilesByDefault(t *testing.T) {
 	assert.True(t, exists)
 
 	// Verify manifest only has 1 file (the non-dotfile)
-	manifest, err := mfer.NewManifestFromFile(fs, testMF)
+	manifest, err := mfer.NewManifestFromFile(&mfer.ManifestFromFileOptions{
+		Path: testMF,
+		Fs:   fs,
+	})
 	require.NoError(t, err)
 	assert.Len(t, manifest.Files(), 1)
 	assert.Equal(t, "file1.txt", manifest.Files()[0].GetPath())
@@ -528,7 +531,10 @@ func TestGenerateWithIncludeDotfiles(t *testing.T) {
 	require.Equal(t, 0, exitCode)
 
 	// Verify manifest has 2 files (including dotfile)
-	manifest, err := mfer.NewManifestFromFile(fs, testMF)
+	manifest, err := mfer.NewManifestFromFile(&mfer.ManifestFromFileOptions{
+		Path: testMF,
+		Fs:   fs,
+	})
 	require.NoError(t, err)
 	assert.Len(t, manifest.Files(), 2)
 }