Typst boxed answers: keep sentence punctuation outside the box

When wrapping a final answer in a framed box mid-sentence (e.g. 'runs in #boxed[$O(n)$ time,] which is...'), any trailing comma or period inside the brackets renders inside the frame and looks wrong. End the box at the bare answer and move any continuation into a new sentence after it. Homework graders and users notice stray punctuation inside boxes.