Reject manifests whose file entries decode far larger than their bytes (closes #123)

Before decoding the manifest, the parser adds up what decoding sets
aside for each file entry, hash, timestamp and MIME type, however short
its encoding, and refuses the manifest once that passes 8 times the
decompressed size; manifests mfer writes come to at most about 7.15
times. Empty entries decoded to about 50 times their size: under 1 KB of
manifest allocated about 500 MB. Fields the decoder does not know are
dropped; kept, they took up to 5 times more. A test refuses entries
counted just over 8 times and loads them just under. The fuzz ceiling
rises from 16 to 20 times the input and decompressed data; seeds of
empty entries and of empty hashes fail it without the fix.

Model: opus-5-5

mfer/deserialize_fuzz_test.go

@@ -54,9 +54,15 @@ func FuzzNewManifestFromReader(f *testing.F) {
 		}
 
 		// It also keeps a few copies of its input. Buffers grow by
-		// copying, so reaching those sizes allocates a few times them in
-		// total: sixteen times the input and the decompressed data leaves
-		// room for that.
+		// copying, so reaching those sizes allocates up to about six times
+		// them in total. Decoding the decompressed data takes up to
+		// maxDecodedGrowth times its size for file entries, hashes,
+		// timestamps and MIME types, and drops fields it does not know.
+		// The strings and bytes it copies out of it, such as many one-byte
+		// values in one hash, take up to about five times more under the
+		// race detector, which pads every small copy to 16 bytes, and about
+		// half that without it. Twenty times the input and the
+		// decompressed data leaves room for all of that.
 		//
 		// The decoder also sets aside a new buffer of one to two times the
 		// window for each frame that asks for a larger window than the
@@ -71,8 +77,9 @@ func FuzzNewManifestFromReader(f *testing.F) {
 		// fails if the decoder accepts windows of twice zstdWindowSize; the
 		// seed whose two frames together exceed MaxDecompressedSize fails
 		// if the decoder decodes them in full instead of stopping at the
-		// declared size.
-		limit := 16*(uint64(len(data))+decompressed) + 24*zstdWindowSize
+		// declared size; the seeds of empty file entries and of a file
+		// entry of empty hashes fail if the parser decodes them.
+		limit := 20*(uint64(len(data))+decompressed) + 24*zstdWindowSize
 
 		allocated := after.TotalAlloc - before.TotalAlloc
 		if allocated > limit {