The ornamental design for stool seat, as shown and described.
FIG. 1 is a front perspective view of the stool seat, the broken lines shown in the drawings are for illustrative purposes only and form no part of the claimed design.
FIG. 2 is a rear perspective view of the stool seat.
FIG. 3 is a front elevational view of the stool seat.
FIG. 4 is a right side elevational view of the stool seat, the left side elevational view being a mirror image thereof.
FIG. 5 is a rear elevational view of the stool seat.