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

src/fesa/math/dense_blas_internal.h

@@ -6,6 +6,7 @@
 #include <cstddef>
 
 #include "fesa/core/status.h"
+#include "fesa/math/matrix.h"
 
 namespace fesa::dense_blas_internal {
 
@@ -13,6 +14,10 @@ namespace fesa::dense_blas_internal {
 /// @return The converted length or an uncategorized overflow failure.
 Result<MKL_INT> ToMklSize(std::size_t size);
 
+/// @brief Computes left-transpose times right through ordinary dense MKL.
+/// @throws std::invalid_argument if the row counts differ.
+Matrix MultiplyTransposeLeft(const Matrix& left, const Matrix& right);
+
 /// @brief Copies contiguous double values through the private BLAS backend.
 /// @param source Readable storage for size values when size is nonzero.
 /// @param size Number of contiguous values to copy.