Refuse a file entry with no hash when loading a manifest (closes #194)

Loading accepted a file entry whose hashes list was empty. check and
fetch then failed the file as a hash mismatch, and freshen left it out
of the rewritten manifest without a word.

NewManifestFromReader now refuses such an entry, naming its path, after
checking the path itself. freshen's addExistingToBuilder returns an
error for an entry with no hash instead of skipping it. The file entry
table in docs/FORMAT.md says readers reject an entry with none.

Model: opus-5-5

internal/cli/freshen.go

@@ -28,9 +28,15 @@ const (
 	scanProgressInterval = 100
 )
 
-// errEntryMissingMtime indicates a manifest entry that carries no
-// modification time where one is required to carry it forward unchanged.
-var errEntryMissingMtime = errors.New("manifest entry has no mtime")
+var (
+	// errEntryMissingMtime indicates a manifest entry that carries no
+	// modification time where one is required to carry it forward unchanged.
+	errEntryMissingMtime = errors.New("manifest entry has no mtime")
+
+	// errEntryMissingHash indicates a manifest entry that carries no hash
+	// to carry forward.
+	errEntryMissingHash = errors.New("manifest entry has no hash")
+)
 
 // FreshenStatus contains progress information for the freshen operation.
 type FreshenStatus struct {
@@ -644,6 +650,8 @@ func addFileToBuilder(
 // Entries reach this path only when recordEntry classified them as
 // unchanged, which requires a recorded mtime, so an absent mtime here is
 // an error rather than something to paper over with the Unix epoch.
+// Loading refuses an entry with no hash; one that got here anyway is an
+// error too, not a file to leave out of the manifest.
 func addExistingToBuilder(b *mfer.Builder, entry *mfer.MFFilePath) error {
 	mtime, ok := entryMtime(entry)
 	if !ok {
@@ -651,7 +659,7 @@ func addExistingToBuilder(b *mfer.Builder, entry *mfer.MFFilePath) error {
 	}
 
 	if len(entry.GetHashes()) == 0 {
-		return nil
+		return fmt.Errorf("%w: %s", errEntryMissingHash, entry.GetPath())
 	}
 
 	err := b.AddFileWithHash(mfer.RelFilePath(entry.GetPath()),