Apply Eigen upstream patch for eigen_memset_helper

Source: https://gitlab.com/libeigen/eigen/-/commit/e246f9cb68d07e8f15c60a2404ac9a625c349223

Author: eddelbuettel

DESCRIPTION

@@ -1,8 +1,8 @@
 Package: RcppEigen
 Type: Package
 Title: 'Rcpp' Integration for the 'Eigen' Templated Linear Algebra Library
-Version: 0.4.9.9-2
-Date: 2026-06-04
+Version: 0.4.9.9-4
+Date: 2026-10-01
 Authors@R: c(person("Doug", "Bates", role = "aut",
                     comment = c(ORCID = "0000-0001-8316-9503")),
              person("Dirk", "Eddelbuettel", role = c("aut", "cre"), email = "edd@debian.org",

inst/include/Eigen/src/Core/Fill.h

@@ -93,8 +93,10 @@ struct eigen_fill_impl<Xpr, /*use_fill*/ true> {
 
 template <typename Xpr>
 struct eigen_memset_helper {
-  static constexpr bool value =
-      std::is_trivially_copyable<typename Xpr::Scalar>::value && eigen_fill_helper<Xpr>::value;
+  using Scalar = typename Xpr::Scalar;
+  static constexpr bool value = std::is_trivially_copyable<Scalar>::value &&
+                                !static_cast<bool>(NumTraits<Scalar>::RequireInitialization) &&
+                                eigen_fill_helper<Xpr>::value;
 };
 
 template <typename Xpr>