Add count_linear_extensions() R function for approximating linear extensions

[ChinmayRout9040895625]

Adds an R interface for counting linear extensions of partial orders via OrderPolytope
volume estimation. Depends on the corresponding volesti C++ changes.

(1) Changes

• src/count_linear_extensions.cpp — Rcpp bridge wrapping the C++ OrderPolytope volume API
• R/count_linear_extensions.R — R wrapper function with roxygen2 documentation
• NAMESPACE — Exported count_linear_extensions

(2)Usage

(i)Chain: exactly 1 linear extension
count_linear_extensions(4, rbind(c(0,1), c(1,2), c(2,3)))
(ii)Antichain: exactly 4! = 24 linear extensions
count_linear_extensions(4, matrix(ncol=2, nrow=0))
(iii)With algorithm choice
count_linear_extensions(4, rbind(c(0,1), c(0,2)), algorithm = "CG")