fix: add backend-clear method for raw 2D arrays

Same pattern as the draw-text array fix. Application code may call
backend-clear with a framebuffer array instead of a backend instance.
The array method clears all cells to default blank state.

src/rendering/framebuffer.lisp

@@ -93,6 +93,11 @@
                               :fg fg :bg bg
                               :bold bold :italic italic :underline underline)))))
 
+(defmethod backend-clear ((fb array))
+  (dotimes (y (array-dimension fb 0))
+    (dotimes (x (array-dimension fb 1))
+      (setf (aref fb y x) (make-cell)))))
+
 (defmethod draw-rect ((fb framebuffer-backend) x y w h &key bg)
   (dotimes (row h)
     (dotimes (col w)