feat(hex8): preserve full high-low state in dynamic element responses

src/fesa/elements/hex8_geometry.cpp

@@ -133,7 +133,7 @@ std::pair<Coefficients, Coefficients> Split(const Coefficients& original,
 }  // namespace
 
 Certificate Certify(const Hex8Element::Coordinates& coordinates,
-                    const Vector* displacement) {
+                    const Vector* displacement, const Vector* displacement_low) {
   std::array<std::array<Interval, 3>, 8> x{};
   for (std::size_t a = 0; a < 8; ++a)
     for (std::size_t i = 0; i < 3; ++i) {
@@ -145,6 +145,11 @@ Certificate Certify(const Hex8Element::Coordinates& coordinates,
         if (!std::isfinite(u)) return Certificate::kNonfinite;
         x[a][i] = Add(x[a][i], {u, u});
       }
+      if (displacement_low) {
+        const auto low = (*displacement_low)[3U * a + i];
+        if (!std::isfinite(low)) return Certificate::kNonfinite;
+        x[a][i] = Add(x[a][i], {low, low});
+      }
       if (!Finite(x[a][i])) return Certificate::kNonfinite;
     }
   Coefficients coefficients{};