feat: Add XOR-based doubly linked list mode for memory efficiency

Implements issue #692 by adding an optional 'xor' mode to DoublyLinkedList
that uses XOR of adjacent node addresses to reduce memory usage from 2
pointers to 1 pointer per node.

Features:
- Add mode parameter ('standard' or 'xor') to DoublyLinkedList.__new__
- Implement XOR node structure with links=['both']
- Maintain id_to_node dictionary for pointer dereferencing
- Refactor all insert/extract methods for XOR mode support
- Override __getitem__, __str__, and search for XOR traversal
- Maintain full backward compatibility with standard mode

Testing:
- Add comprehensive test suite for XOR mode
- Test parity between standard and XOR modes
- Test edge cases (empty, single, large lists)
- All 114 existing tests still pass

Fixes #692

Author: Sreevarsh-Mahesh