The ornamental design for a stair tread, as shown and described.
FIG. 1 provides a top front perspective view of a stair tread.
FIG. 2 provides a front elevation view of a stair tread.
FIG. 3 provides a back elevation view of a stair tread.
FIG. 4 provides a right side elevation view of a stair tread.
FIG. 5 provides a left side elevation view of a stair tread.
FIG. 6 provides a top elevation view of a stair tread.
FIG. 7 provides a bottom elevation view of a stair tread; and,
FIG. 8 provides a perspective view of a stair tread installed on a staircase.
The broken lines shown in FIGS. 3 and 7 are for the purpose of illustrating portions of the stair tread and form no part of the claimed design. The broken lines showing a stair in FIG. 8 are for the purpose of illustrating environment and form no part of the claimed design.