v0.5.5: categorical Nan handling, gputree and sagemaker support

Version 0.5.5:

Bug Fixes

• Handle missing values in categorical features by surfacing a "NaN" option in inputs and normalizing NaN selections back to real missing values.
• Add tests covering categorical NaN handling for both merged and unmerged input paths.
• Preserve categorical dtypes during permutation importance shuffles and PDP grid generation to prevent dtype-related model errors (e.g., LightGBM).
• Align categorical/boolean dtypes for user-provided X_row inputs and add dtype alignment tests.

Improvements

• Add support for GPU Tree SHAP explainers via shap='gputree' (requires CUDA-enabled SHAP).
• Add SageMaker Studio support: auto-detect environment, apply proxy prefixes, and CLI flags for overrides.