Default the manifest to index.mf and keep it out of its own listing (closes #100)

mfer gen now writes index.mf instead of .index.mf, the name fetch
requests and the README calls the standard filename. Given a
directory, check, freshen, list and export look only for index.mf;
.index.mf is no longer recognized. Because index.mf is not hidden, gen
and freshen leave out of their own listing the manifest they write and
a temp file an interrupted run left beside it, matched by file
identity (os.SameFile) however the path is spelled. The scanner takes
these as a plain ExcludePaths list; manifestTempPath alone names the
temp file, for both writes, both skips and the tests.

Model: opus-5-5

mfer/checker_test.go

@@ -21,6 +21,8 @@ const (
 	testExistsFile   = "exists.txt"
 	testManifestPath = "/manifest.mf"
 	testDataDir      = "/data"
+	// testDataManifestPath is a manifest kept inside the checked tree.
+	testDataManifestPath = testDataDir + "/index.mf"
 )
 
 func TestStatusString(t *testing.T) {
@@ -464,7 +466,7 @@ func TestFindExtraFilesSkipsManifestAndDotfiles(t *testing.T) {
 	manifestFiles := map[string][]byte{
 		testFile1: []byte("in manifest"),
 	}
-	createTestManifest(t, fs, "/data/.index.mf", manifestFiles)
+	createTestManifest(t, fs, testDataManifestPath, manifestFiles)
 	createFilesOnDisk(t, fs, map[string][]byte{
 		testFile1: []byte("in manifest"),
 	})
@@ -476,7 +478,7 @@ func TestFindExtraFilesSkipsManifestAndDotfiles(t *testing.T) {
 	require.NoError(t, afero.WriteFile(fs, "/data/extra.txt", []byte("extra"), 0o644))
 
 	chk, err := NewChecker(&CheckerOptions{
-		ManifestPath: "/data/.index.mf",
+		ManifestPath: testDataManifestPath,
 		BasePath:     testDataDir,
 		Fs:           fs,
 	})
@@ -491,7 +493,7 @@ func TestFindExtraFilesSkipsManifestAndDotfiles(t *testing.T) {
 		extras = append(extras, r)
 	}
 
-	// Should only report extra.txt, not .hidden, .config/settings, or .index.mf
+	// Should only report extra.txt, not .hidden, .config/settings, or index.mf
 	for _, e := range extras {
 		t.Logf("extra: %s", e.Path)
 	}
@@ -656,7 +658,7 @@ func TestFindExtraFilesSkipsDotfiles(t *testing.T) {
 	files := map[string][]byte{
 		testFile1: []byte("in manifest"),
 	}
-	createTestManifest(t, fs, "/data/.index.mf", files)
+	createTestManifest(t, fs, testDataManifestPath, files)
 	createFilesOnDisk(t, fs, files)
 
 	// Add dotfiles and manifest file on disk
@@ -666,7 +668,7 @@ func TestFindExtraFilesSkipsDotfiles(t *testing.T) {
 		afero.WriteFile(fs, "/data/.git/config", []byte("git config"), 0o644))
 
 	chk, err := NewChecker(&CheckerOptions{
-		ManifestPath: "/data/.index.mf",
+		ManifestPath: testDataManifestPath,
 		BasePath:     testDataDir,
 		Fs:           fs,
 	})
@@ -695,11 +697,11 @@ func TestFindExtraFilesSkipsManifestFile(t *testing.T) {
 	files := map[string][]byte{
 		testFile1: []byte("content"),
 	}
-	createTestManifest(t, fs, "/data/index.mf", files)
+	createTestManifest(t, fs, testDataManifestPath, files)
 	createFilesOnDisk(t, fs, files)
 
 	chk, err := NewChecker(&CheckerOptions{
-		ManifestPath: "/data/index.mf",
+		ManifestPath: testDataManifestPath,
 		BasePath:     testDataDir,
 		Fs:           fs,
 	})

mfer/scanner.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"io"
 	"io/fs"
+	"os"
 	"path"
 	"path/filepath"
 	"strings"
@@ -57,6 +58,8 @@ type ScannerOptions struct {
 	SigningOptions *SigningOptions
 	// Seed, if set, derives a deterministic UUID from this seed.
 	Seed string
+	// ExcludePaths lists files to leave out of the listing, matched with os.SameFile.
+	ExcludePaths []string
 }
 
 // FileEntry represents a file that has been enumerated.
@@ -75,6 +78,7 @@ type Scanner struct {
 	totalBytes FileSize // cached sum of all file sizes
 	options    *ScannerOptions
 	fs         afero.Fs
+	excluded   []fs.FileInfo // the files named in ExcludePaths that exist
 }
 
 // NewScanner creates a new Scanner with default options.
@@ -93,11 +97,22 @@ func NewScannerWithOptions(opts *ScannerOptions) *Scanner {
 		fs = afero.NewOsFs()
 	}
 
-	return &Scanner{
+	s := &Scanner{
 		files:   make([]*FileEntry, 0),
 		options: opts,
 		fs:      fs,
 	}
+
+	// A path that cannot be stat'd, normally because no file is there,
+	// needs no leaving out.
+	for _, p := range opts.ExcludePaths {
+		info, err := s.fs.Stat(p)
+		if err == nil {
+			s.excluded = append(s.excluded, info)
+		}
+	}
+
+	return s
 }
 
 // EnumerateFile adds a single file to the scanner, calling stat() to get metadata.
@@ -435,6 +450,12 @@ func (s *Scanner) enumerateFileWithInfo(
 		info = realInfo
 	}
 
+	for _, excluded := range s.excluded {
+		if os.SameFile(info, excluded) {
+			return nil
+		}
+	}
+
 	entry := &FileEntry{
 		Path:    RelFilePath(cleanPath),
 		AbsPath: AbsFilePath(absPath),