I claim the ornamental design for a bed foundation, as shown and described.
FIG. 1 is a perspective view of a bed foundation showing the present design in a position of use;
FIG. 2 is an enlarged detail view of the bed foundation of FIG. 1;
FIG. 3 is a top view thereof;
FIG. 4 is a front elevational view thereof;
FIG. 5 is a left side elevational view thereof;
FIG. 6 is a perspective view of a second embodiment of the bed foundation of the present design; and,
FIG. 7 is a top view thereof.
The broken lines in FIG. 1 depict portions of the bed foundation that are not included in the claim and indicate the outer limits to the claim. The broken lines in FIGS. 2-7 represent the boundaries of the claim and form no part thereof.