fix: use os.ErrNotExist instead of errors.New() for file-not-found detection in checkers

errors.Is(err, errors.New("file does not exist")) can never match because
errors.New creates a unique value each time. Replace with os.ErrNotExist
which is the standard Go sentinel for file-not-found errors.

Fixes both mfer/checker.go and internal/checker/checker.go.

mfer/builder_test.go

@@ -92,6 +92,29 @@ func TestBuilderBuild(t *testing.T) {
 	assert.True(t, strings.HasPrefix(buf.String(), MAGIC))
 }
 
+func TestNewTimestampFromTimeExtremeDate(t *testing.T) {
+	// Regression test: newTimestampFromTime used UnixNano() which panics
+	// for dates outside ~1678-2262. Now uses Nanosecond() which is safe.
+	tests := []struct {
+		name string
+		time time.Time
+	}{
+		{"zero time", time.Time{}},
+		{"year 1000", time.Date(1000, 1, 1, 0, 0, 0, 0, time.UTC)},
+		{"year 3000", time.Date(3000, 1, 1, 0, 0, 0, 123456789, time.UTC)},
+		{"unix epoch", time.Unix(0, 0)},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			// Should not panic
+			ts := newTimestampFromTime(tt.time)
+			assert.Equal(t, tt.time.Unix(), ts.Seconds)
+			assert.Equal(t, int32(tt.time.Nanosecond()), ts.Nanos)
+		})
+	}
+}
+
 func TestBuilderBuildEmpty(t *testing.T) {
 	b := NewBuilder()
 

mfer/checker.go

@@ -224,12 +224,7 @@ func (c *Checker) checkFile(entry *MFFilePath, checkedBytes *FileSize) Result {
 	// Check if file exists
 	info, err := c.fs.Stat(absPath)
 	if err != nil {
-		if errors.Is(err, afero.ErrFileNotFound) || errors.Is(err, errors.New("file does not exist")) {
-			return Result{Path: relPath, Status: StatusMissing, Message: "file not found"}
-		}
-		// Check for "file does not exist" style errors
-		exists, _ := afero.Exists(c.fs, absPath)
-		if !exists {
+		if errors.Is(err, os.ErrNotExist) || errors.Is(err, afero.ErrFileNotFound) {
 			return Result{Path: relPath, Status: StatusMissing, Message: "file not found"}
 		}
 		return Result{Path: relPath, Status: StatusError, Message: err.Error()}

mfer/checker_test.go

@@ -381,6 +381,39 @@ func TestCheckSubdirectories(t *testing.T) {
 	assert.Equal(t, 3, okCount)
 }
 
+func TestCheckMissingFileDetectedWithoutFallback(t *testing.T) {
+	// Regression test: errors.Is(err, errors.New("...")) never matches because
+	// errors.New creates a new value each time. The fix uses os.ErrNotExist instead.
+	fs := afero.NewMemMapFs()
+	files := map[string][]byte{
+		"exists.txt":  []byte("here"),
+		"missing.txt": []byte("not on disk"),
+	}
+	createTestManifest(t, fs, "/manifest.mf", files)
+	// Only create one file on disk
+	createFilesOnDisk(t, fs, "/data", map[string][]byte{
+		"exists.txt": []byte("here"),
+	})
+
+	chk, err := NewChecker("/manifest.mf", "/data", fs)
+	require.NoError(t, err)
+
+	results := make(chan Result, 10)
+	err = chk.Check(context.Background(), results, nil)
+	require.NoError(t, err)
+
+	statusCounts := map[Status]int{}
+	for r := range results {
+		statusCounts[r.Status]++
+		if r.Status == StatusMissing {
+			assert.Equal(t, RelFilePath("missing.txt"), r.Path)
+		}
+	}
+	assert.Equal(t, 1, statusCounts[StatusOK], "one file should be OK")
+	assert.Equal(t, 1, statusCounts[StatusMissing], "one file should be MISSING")
+	assert.Equal(t, 0, statusCounts[StatusError], "no files should be ERROR")
+}
+
 func TestCheckEmptyManifest(t *testing.T) {
 	fs := afero.NewMemMapFs()
 	// Create manifest with no files

mfer/serialize.go

@@ -16,11 +16,10 @@ import (
 const MAGIC string = "ZNAVSRFG"
 
 func newTimestampFromTime(t time.Time) *Timestamp {
-	out := &Timestamp{
+	return &Timestamp{
 		Seconds: t.Unix(),
-		Nanos:   int32(t.UnixNano() - (t.Unix() * 1000000000)),
+		Nanos:   int32(t.Nanosecond()),
 	}
-	return out
 }
 
 func (m *manifest) generate() error {