We claim the ornamental design for a seat assembly, as shown and described.
FIG. 1 is a perspective view of the seat assembly on a stool showing a right side, a top and a front.
FIG. 2 is a top plan view of the seat assembly of FIG. 1.
FIG. 3 is a front elevational view.
FIG. 4 is a rear elevational view.
FIG. 5 is a left side elevational view, the right side being a mirror image; and,
FIG. 6 is a bottom plan view.
The broken line showings of the mounting portions and the ribs of the seat assembly form no part of the claimed design. The broken line showing of a portion of the stool illustrates the environment of the claimed design and forms no part thereof