Circular buffer

Implement a fixed-capacity circular buffer (ring) of size `cap`. Operations: ['enqueue', x] inserts at the rear and returns True, or False if full; ['dequeue'] removes from the front and returns the value, or None if empty; ['front'] returns the front value or None; ['rear'] returns the rear value or None. Given cap and an op list, return the list of results for enqueue/dequeue/front/rear (in op order). Use None for empty results.