Require a SHA-256 hash in every manifest entry (closes #224)

Loading accepted an entry whose hashes held no SHA-256 multihash, and
check and fetch, which compute only SHA-256, then reported the intact
file as a hash mismatch. Loading now refuses such an entry, naming its
path. AddFileWithHash accepts only a SHA-256 multihash, and freshen
carries an unchanged entry's SHA-256 hash over instead of its first
hash. SHA256Multihash, new in package mfer, finds that hash for both.
docs/FORMAT.md states the rule for writers and readers. Two loading
tests whose entries held an empty hash now give each entry a SHA-256
hash; the decoding-size test's path lengths change with it.

Model: opus-5-5

internal/cli/freshen.go

@@ -33,9 +33,9 @@ var (
 	// 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")
+	// errEntryMissingHash indicates a manifest entry that carries no
+	// SHA-256 hash to carry forward.
+	errEntryMissingHash = errors.New("manifest entry has no SHA-256 hash")
 )
 
 // errSignKeyNeeded indicates a signed manifest given to freshen with no
@@ -699,21 +699,24 @@ 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.
+// Only the entry's SHA-256 hash is carried over, wherever the entry lists
+// it, since check and fetch compare with no other. Loading refuses an
+// entry with no SHA-256 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 {
 		return fmt.Errorf("%w: %s", errEntryMissingMtime, entry.GetPath())
 	}
 
-	if len(entry.GetHashes()) == 0 {
+	hash := mfer.SHA256Multihash(entry)
+	if hash == nil {
 		return fmt.Errorf("%w: %s", errEntryMissingHash, entry.GetPath())
 	}
 
 	err := b.AddFileWithHash(mfer.RelFilePath(entry.GetPath()),
 		mfer.FileSize(entry.GetSize()), mfer.ModTime(mtime),
-		fs.FileMode(entry.GetMode()), entry.GetHashes()[0].GetMultiHash())
+		fs.FileMode(entry.GetMode()), hash)
 	if err != nil {
 		return fmt.Errorf(
 			"manifest entry %s: %w (regenerate the manifest with mfer generate)",