The ornamental design for a seat, as shown and described.
FIG. 1 is a left front perspective view of a seat showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a side elevational view, the opposite side being a mirror image thereof; and,
FIG. 4 is a rear view thereof.
The views of the seat not shown form no part of the claimed design. The broken lines depicting nail heads represent environmental elements only and form no part of the claimed design.