Record file mode in the manifest, 0000 unless asked (closes #161)

MFFilePath gains mode (field 304): a file's permission bits, 0777 at
most, or 0000, meaning none recorded. gen and freshen record real modes
only with --include-permissions (ScannerOptions.IncludePermissions); the
builder keeps only mode.Perm(), so setuid, setgid and sticky are never
written. list -l and export show the mode in octal. check reports
MODE_MISMATCH for a recorded mode other than 0000 the file lacks. fetch
refuses a manifest with a mode above 0777 before requesting any file,
sets only the permission bits of each recorded mode on the files it
writes, and downloads again a present file whose mode differs. The decoding-cost bound counts a file
entry at 176 bytes, up from 160.

Model: opus-5-5

mfer/deserialize_path_test.go

@@ -119,11 +119,11 @@ func TestDeserializeRejectsInvalidEntryPaths(t *testing.T) {
 }
 
 // Entries of a path, an empty hash, an empty MIME type and empty modification
-// and change times are counted at 416 bytes each (160 + 112 + 16 + 64 + 64)
-// and take 16 bytes plus the path to encode. A 35-character path makes that
-// 51 bytes, about 8.2 times: refused, and leaving any one of the five
-// uncounted, even the MIME type, brings it under 8. A 37-character path makes
-// it 53 bytes, about 7.8 times: loaded.
+// and change times are counted at 432 bytes each (176 + 112 + 16 + 64 + 64)
+// and take 16 bytes plus the path to encode. A 37-character path makes that
+// 53 bytes, about 8.2 times: refused, and leaving any one of the five
+// uncounted, even the MIME type, brings it under 8. A 39-character path makes
+// it 55 bytes, about 7.9 times: loaded.
 func TestDeserializeRefusesEntriesThatDecodeTooLarge(t *testing.T) {
 	t.Parallel()
 
@@ -131,8 +131,8 @@ func TestDeserializeRefusesEntriesThatDecodeTooLarge(t *testing.T) {
 		pathLen int
 		refused bool
 	}{
-		{35, true},
-		{37, false},
+		{37, true},
+		{39, false},
 	}
 
 	for _, tt := range tests {
@@ -215,7 +215,7 @@ func TestDeserializeLoadsDensestManifest(t *testing.T) {
 	const files = 10000
 	for i := range files {
 		name := RelFilePath(strconv.FormatInt(int64(i), 36))
-		require.NoError(t, b.AddFileWithHash(name, 0, ModTime(time.Unix(0, 0)), hash))
+		require.NoError(t, b.AddFileWithHash(name, 0, ModTime(time.Unix(0, 0)), 0, hash))
 	}
 
 	var buf bytes.Buffer
@@ -233,7 +233,7 @@ func TestDeserializeValidManifestRoundTrips(t *testing.T) {
 	require.NoError(t, err)
 
 	b := NewBuilder()
-	require.NoError(t, b.AddFileWithHash("dir/file.txt", 123, ModTime{}, hash))
+	require.NoError(t, b.AddFileWithHash("dir/file.txt", 123, ModTime{}, 0, hash))
 
 	var buf bytes.Buffer
 	require.NoError(t, b.Build(context.Background(), &buf))