Circular buffer

Implement a fixed-size circular (ring) buffer of capacity `capacity`. Process operations: ["push", x] appends x; if the buffer is full, the oldest element is overwritten (dropped) to make room. ["pop"] removes and conceptually returns the oldest element (a no-op if empty). ["snapshot"] yields the current buffer contents from oldest to newest. Return a list containing one entry per snapshot operation, each being the list of current contents oldest-to-newest. capacity is a positive integer.