Add explicit two- and three-group risk stratification for v2.3.0

[raptor419]

This PR adds three-group risk stratification while preserving the existing two-group method.

Changes

• Add n_groups=2 as the default with legacy-compatible behavior.
• Add n_groups=3 for low, middle, and high-risk groups using two thresholds.
• Support fixed or adaptively learned threshold pairs.
• Support default, protective, and high_risk effects with three groups.
• Keep exactly two thresholds throughout three-group evolution.
• Add three-group prediction, transformation, reporting, plotting, and group-extraction support.
• Add pairwise log-rank and Kruskal-Wallis fitness calculations.
• Remove the unreleased multi_thresholding parameter and automatic group-count selection.
• Update documentation and bump the package version to 2.3.0.