fix(io): read the whole slice in ReadBytesShort

[0xShadowX]

io.ReadBytesShort reads the payload with a single reader.Read call. io.Reader allows Read to return fewer bytes than requested with a nil error, so with a reader that does short reads the rest of the slice stays zero and no error is returned:

var buf bytes.Buffer
gnarkio.WriteBytesShort(challenge, &buf) // 32-byte challenge
got, n, err := gnarkio.ReadBytesShort(iotest.OneByteReader(&buf))
// err == nil, n == 2, got == ab000000...00

This happens in practice with bufio.Reader at a buffer boundary, pipes or network streams. A stream that ends in the middle of the payload isn't reported either. The function is used by the groth16 mpcsetup Phase1/Phase2 ReadFrom on all curves to read the contribution challenge, so a contribution read that way would carry a wrong challenge.

The fix uses io.ReadFull, which keeps reading until the slice is full and returns io.ErrUnexpectedEOF on a truncated stream.

Tests:

go test -count=1 -run TestReadBytesShort ./io/
go test -short ./backend/groth16/bn254/mpcsetup/ ./backend/groth16/bls12-377/mpcsetup/ ./backend/groth16/bls12-381/mpcsetup/ ./backend/groth16/bw6-761/mpcsetup/

TestReadBytesShortPartialReads (one byte per Read) and TestReadBytesShortTruncated both fail on master and pass with the change.

• regression tests added
• no generated files touched