Proportional Trimmed Mean // Computes a proportional trimmed mean that resolves the integer truncation problem of base R's mean(..., trim). When k = trim * n is non-integer, a fractional discount (1 - delta) is applied to boundary observations, where delta = k - floor(k). The resulting estimator is continuous in alpha for any fixed n, syntactically identical to mean(..., trim), and compatible with the 'Statgraphics' implementation. See Gaviria Chaverra (2026) doi:10.32614/CRAN.package.sgmean.