AddFileWithHash rejects hashes that are not multihashes (closes #129)

AddFileWithHash took any non-empty bytes as a hash, so the builder could
write a manifest that mfer refuses to load. It now decodes the hash with
go-multihash and also requires a digest of at least 32 bytes, the SHA-256
length the reader's decoding-cost limit assumes: a valid but shorter
multihash, such as an empty identity hash or SHA-1, still makes a
manifest of one-character paths too costly to load. When mfer freshen
meets such a hash in an existing manifest, its error names the manifest
entry and says to regenerate the manifest with mfer generate. Test
fixtures whose stand-in hashes were not valid multihashes now use a
SHA-256 multihash.

Model: opus-5-5

mfer/deserialize_path_test.go

@@ -13,6 +13,7 @@ import (
 
 	"github.com/google/uuid"
 	"github.com/klauspost/compress/zstd"
+	"github.com/multiformats/go-multihash"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 	"google.golang.org/protobuf/encoding/protowire"
@@ -205,7 +206,8 @@ func TestDeserializeDropsUnknownFields(t *testing.T) {
 func TestDeserializeLoadsDensestManifest(t *testing.T) {
 	t.Parallel()
 
-	hash := make([]byte, 34) // multihash: 2-byte prefix + 32-byte SHA-256
+	hash, err := multihash.Encode(make([]byte, sha256.Size), multihash.SHA2_256)
+	require.NoError(t, err)
 
 	b := NewBuilder()
 	b.SetIncludeTimestamps(true)
@@ -227,7 +229,8 @@ func TestDeserializeLoadsDensestManifest(t *testing.T) {
 func TestDeserializeValidManifestRoundTrips(t *testing.T) {
 	t.Parallel()
 
-	hash := make([]byte, 34) // multihash: 2-byte prefix + 32-byte SHA-256
+	hash, err := multihash.Encode(make([]byte, sha256.Size), multihash.SHA2_256)
+	require.NoError(t, err)
 
 	b := NewBuilder()
 	require.NoError(t, b.AddFileWithHash("dir/file.txt", 123, ModTime{}, hash))