v0.26.2

fix: Bound the declared string length in ReadString before allocating

ReadString allocated a buffer sized by the declared length without checking it against the file. Mirror the existing SkipReadingString / _validateCountWithRemaining guards: reject a length above math.MaxInt64, and, when the file size is known, a length beyond the bytes remaining in the file.

Adds TestReadStringRejectsOversizedLength.