dsh-tool-stat: DSH Statistics Tool Plugin: Descriptive Statistics, Percentiles, Frequency Distributions, and Correlation Calculations

`dsh-tool-stat` is a statistical tool plugin for the DSH workflow, designed to transform numerical computations from model-based mental calculations into reproducible, structured operations. This plugin, built on pure functions and deterministic execution with no dependency on third-party numerical libraries, offers four core functions: `describe` computes descriptive statistics such as mean, variance, and quartiles; `percentile` calculates percentiles using linear interpolation; `frequency` generates frequency distributions and computes proportions; and `correlation` calculates Pearson or Spearman correlation coefficients, returning a specific status instead of an error value in cases of zero variance. The plugin enforces strict security constraints, rejecting NaN/Infinity values, normalizing `-0`, and limiting the number of observations (1 to 100,000) and percentile requests (up to 100).