Two bugs caused pawns to teleport back to the yard without capture:

1. Triple-six penalty blindly sent the last-moved piece to pos=-1
   regardless of whether it was resting on a safe square. Now the
   penalty only applies if the piece is NOT on a safe square.

2. captureAt() checked if the DESTINATION was safe but didn't guard
   against the victim's own position being safe. Added a per-piece
   safety check so a pawn sitting on any safe square (star squares,
   colored start squares) can never be captured under any circumstance.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>