revert: remove unrelated Hermite interpolation modifications

Author: Harsh Singh

lib/OrdinaryDiffEqRosenbrock/src/rosenbrock_perform_step.jl

@@ -130,12 +130,10 @@ end
                 integrator.EEst = max(EEst, integrator.EEst)
             end
         else
-            # No H matrix: compute Hermite interpolation coefficients
-            # k[1] = dt*f₀ - (u - uprev), k[2] = 2*(u - uprev) - dt*(f₀ + f₁)
-            f1 = f(u, p, t + dt)
+            # No H matrix: set k[1]=f(uprev), k[2]=f(u_new) for Hermite interpolation
+            integrator.k[1] = fsalfirst_cache
+            integrator.k[2] = f(u, p, t + dt)
             OrdinaryDiffEqCore.increment_nf!(integrator.stats, 1)
-            integrator.k[1] = @.. dt * fsalfirst_cache - (u - uprev)
-            integrator.k[2] = @.. 2 * (u - uprev) - dt * (fsalfirst_cache + f1)
         end
     end
 
@@ -294,12 +292,11 @@ end
                 integrator.EEst = max(EEst, integrator.EEst)
             end
         else
-            # No H matrix: compute Hermite interpolation coefficients
-            # k[1] = dt*f₀ - (u - uprev), k[2] = 2*(u - uprev) - dt*(f₀ + f₁)
+            # No H matrix: set k[1]=fsalfirst, k[2]=f(u_new) for interpolation
+            integrator.k[1] .= cache.fsalfirst
             f(du, u, p, t + dt)
             OrdinaryDiffEqCore.increment_nf!(integrator.stats, 1)
-            @.. integrator.k[1] = dt * cache.fsalfirst - (u - uprev)
-            @.. integrator.k[2] = 2 * (u - uprev) - dt * (cache.fsalfirst + du)
+            integrator.k[2] .= du
         end
     end
     cache.linsolve = linres.cache