Limit how much fetch and check read for a manifest or a file (closes #168)

NewManifestFromReader reads at most one byte past MaxManifestSize, a new
constant of 258 MiB: the 256 MiB decompressed limit grown by zstd's worst
case of 1/256, plus 1 MiB for the signature, the signing key and the
other outer fields. It refuses a larger manifest. fetch, and check given
a URL, stop downloading a manifest one byte past the same size and report
it as too large; tests lower that size to keep their memory small. fetch
stops reading a file one byte past its listed size, so a longer body ends
in the size mismatch at once instead of filling the disk. docs/FORMAT.md
states the limit and gives the decompressed limit as 256 MiB, the size
the code uses.

Model: opus-5-5

mfer/constants.go

@@ -8,10 +8,17 @@ const (
 	ReleaseDate = "2025-12-17"
 
 	// MaxDecompressedSize is the maximum allowed size of decompressed manifest
-	// data (256 MB). This prevents decompression bombs from consuming excessive
+	// data (256 MiB). This prevents decompression bombs from consuming excessive
 	// memory.
 	MaxDecompressedSize int64 = 256 * 1024 * 1024
 
+	// MaxManifestSize is the largest manifest file mfer reads (258 MiB).
+	// zstd's worst case grows data it cannot compress by 1/256, so an inner
+	// message of MaxDecompressedSize compresses to at most 257 MiB; the
+	// last MiB is room for the signature, the signing key and the other
+	// outer fields.
+	MaxManifestSize = MaxDecompressedSize + MaxDecompressedSize/256 + 1<<20
+
 	// zstdWindowSize is the zstd window zstd.SpeedBestCompression gives mfer's writer.
 	zstdWindowSize = 8 << 20