Optimize getitem

[p1c2u]

This pull request improves the performance and correctness of the __getitem__ method in AccessorPath by optimizing traversal logic, and adds comprehensive tests and benchmarks to verify and measure these improvements.

Core logic and performance improvements

• Optimized the AccessorPath.__getitem__ method in pathable/paths.py to minimize redundant traversals and improve efficiency by using direct traversal helpers when available, with compatibility fallbacks for other accessors.

Testing enhancements

• Added a new CountingTraversableAccessor class in tests/unit/test_paths.py to track the number of traversal operations, enabling precise testing of traversal efficiency.
• Introduced a test class TestAccessorPathGetItemPerformance to ensure that only a single traversal occurs when accessing a leaf value, validating the performance improvement.

Benchmarking

• Expanded tests/benchmarks/bench_lookup.py to include benchmarks for __getitem__ operations on both leaf and branch nodes, allowing performance measurement of the new logic.

Type consistency

• Updated type annotations in MockTraversableAccessor to use Mapping[Any, Any] for improved consistency and generality in tests.