Mangala skips the opponent's store, but Board.move dropped a stone in
every pit and store. Moves with enough stones to pass it gave the
opponent a stone and ended one pit too early, so captures and extra
turns were decided on the wrong pit.
Board.getSowingPath now computes the pits a move sows into, and
Board.move and core's getSowingPath (the web board's move preview)
both use it. Game step indexes are now always within the board.
