perf(math): route ordinary products through MKL and preserve arithmetic

src/fesa/math/dense_blas_internal.cpp

@@ -5,6 +5,17 @@
 
 namespace fesa::dense_blas_internal {
 
+Matrix MultiplyTransposeLeft(const Matrix& left, const Matrix& right) {
+  if (left.Rows() != right.Rows()) {
+    throw std::invalid_argument{"Transpose product dimensions differ."};
+  }
+  Matrix transposed{left.Columns(), left.Rows()};
+  for (std::size_t row = 0; row < left.Rows(); ++row)
+    for (std::size_t column = 0; column < left.Columns(); ++column)
+      transposed(column, row) = left(row, column);
+  return transposed.Multiply(right);
+}
+
 Result<MKL_INT> ToMklSize(const std::size_t size) {
   if (size > static_cast<std::size_t>((std::numeric_limits<MKL_INT>::max)())) {
     return Result<MKL_INT>::Failure(Status::Failure(