Minimal Base64 sample: preserve the byte that matters

To reproduce a Base64 problem, reduce the input to the smallest harmless string that still fails. Keep the feature responsible for the mismatch: a non-ASCII character, newline, null byte, or missing padding. State whether the input is text or binary and which encoding was used. Show the encoded value and expected decoded bytes, not just a screenshot of the converter. A compact round-trip example makes it easier to distinguish an encoding issue from a problem in the decoded format.