Fuzz NewManifestFromReader and cap the zstd decoder (closes #65)

FuzzNewManifestFromReader fails when the parser returns both or neither
of a manifest and an error, or allocates more than a fixed multiple of
its input and the decompressed data it may read, plus room for the
decoder's window buffers. make test runs the seed corpus; make fuzz
fuzzes for one minute.

Parser bug: MaxDecompressedSize did not bound decompression; the zstd
decoder decoded a payload under 128 KiB in full before the LimitReader
read any of it. It now decodes only what the LimitReader reads and
refuses windows over the 8 MiB mfer writes with. Seeds: a frame claiming
8 GiB, two frames together over the limit, empty frames with growing
windows.

Model: opus-5-5

mfer/builder_test.go

@@ -4,6 +4,7 @@ package mfer
 import (
 	"bytes"
 	"context"
+	"fmt"
 	"strings"
 	"testing"
 	"time"
@@ -349,6 +350,30 @@ func TestBuilderBuildRoundTrip(t *testing.T) {
 	}
 }
 
+// A manifest whose payload is larger than one zstd block (128 KiB) is
+// written as a frame asking for the writer's whole window, zstdWindowSize,
+// which is the most the parser accepts.
+func TestBuilderBuildRoundTripLargeManifest(t *testing.T) {
+	t.Parallel()
+
+	hash := make([]byte, 34) // multihash: 2-byte prefix + 32-byte SHA-256
+
+	b := NewBuilder()
+
+	for i := range 4000 {
+		path := RelFilePath(fmt.Sprintf("dir/file-%05d.txt", i))
+		require.NoError(t, b.AddFileWithHash(path, FileSize(i), ModTime{}, hash))
+	}
+
+	var buf bytes.Buffer
+	require.NoError(t, b.Build(context.Background(), &buf))
+
+	m, err := NewManifestFromReader(&buf)
+	require.NoError(t, err)
+	require.Greater(t, m.pbOuter.GetSize(), int64(128<<10))
+	assert.Len(t, m.Files(), 4000)
+}
+
 func TestNewManifestFromReaderInvalidMagic(t *testing.T) {
 	t.Parallel()