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/constants.go

@@ -17,4 +17,27 @@ const (
 
 	// uuidLength is the length in bytes of a binary UUID.
 	uuidLength = 16
+
+	// Numbers in mf.proto of MFFile.files and of the MFFilePath fields
+	// that decoding sets aside a fixed amount of memory for.
+	filesFieldNumber    = 101
+	hashesFieldNumber   = 3
+	mimeTypeFieldNumber = 301
+	mtimeFieldNumber    = 302
+	ctimeFieldNumber    = 303
+
+	// Bytes decoding sets aside for each file entry, hash, timestamp and
+	// MIME type, however short its encoding. checkDecodedSize refuses an
+	// inner message for which these add up to more than maxDecodedGrowth
+	// times its size.
+	decodedFileEntrySize = 160
+	decodedHashSize      = 112
+	decodedTimestampSize = 64
+	decodedMIMETypeSize  = 16
+
+	// Each file entry mfer writes holds a path of at least one byte, a
+	// 34-byte SHA-256 multihash and a modification time: at least 47 bytes,
+	// counted at 336. So its manifests add up to at most about 7.15 times
+	// their size, and this limit is about 12% above that.
+	maxDecodedGrowth = 8
 )