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.

org/framebuffer.org

@@ -382,6 +382,18 @@ framebuffer without wrapping it in a ~framebuffer-backend~.
                               :bold bold :italic italic :underline underline)))))
 #+end_src
 
+*** backend-clear (raw array)
+
+Allow clearing a raw 2D framebuffer array directly (same type as returned by
+~make-framebuffer~). Resets all cells to blank defaults.
+
+#+begin_src lisp :tangle ../src/rendering/framebuffer.lisp
+(defmethod backend-clear ((fb array))
+  (dotimes (y (array-dimension fb 0))
+    (dotimes (x (array-dimension fb 1))
+      (setf (aref fb y x) (make-cell)))))
+#+end_src
+
 *** draw-rect
 
 Fill a rectangular region with space characters and an optional background