The ornamental design for a continuousspring, as shown and described.
FIG. 1 is a perspective view of a top portion of the continuous spring illustrating our new design;
FIG. 2 is a top view thereof;
FIG. 3 is a bottom view thereof;
FIG. 4 is front view thereof; the rear view being identical; and,
FIG. 5 is a right side view thereof, the left side being identical.
Any elements shown in broken lines form no part of the claimed invention, and are instead included to show environmental aspects of the claimed invention. The continuous spring is shown with symbolic drafting cut lines to indicate the bounds of the claimed design, and the cut lines form no part of the claimed design.