Add a \break after \hfill in the proof-by-cases.

Add a \break after \hfill in the proof-by-cases.

If a case label was short and the text that followed was short, it was
possible that the text in "Case 1 (label): text" would wind up
right-aligned rather than on a new line. Adding a \break directly
after the \hfill seems to correct this.