The ornamental design for a stadiumseat, as shown and described.
FIG. 1 is a perspective view of a stadium seat design showing my new design.
FIG. 2 is a front view of a stadium seat.
FIG. 3 is a rear view of a stadium seat.
FIG. 4 is a left side view of a stadium seat.
FIG. 5 is a right side view of a stadium seat.
FIG. 6 is a top view of a stadium seat.
FIG. 7 is a bottom view of a stadium seat; and,
FIG. 8 is a perspective view thereof, showing the stadium seat in a folded state of use.
Any shading and cross-hatching are not features of the design but are utilized to illustrate the surface contours of the stadium seat design in the drawings.