Implement a Circular Doubly Linked List

Question

## Problem

Implement a circular doubly linked list with `insert`, `delete`, and `traverse`. AMD reportedly probes pointer manipulation here.

## Example
```
insert at head/tail, delete arbitrary node, traverse forward and backward
```

## Constraints
- Handle empty list and single-node edge cases

## What AMD looks for
Correct pointer rewiring with no leaks or dangling references.