rtx-cfd legacy GPU tests: the two point-source Poisson tests expect the sign of ∇²φ = f

crates/specialized/rtx-cfd/tests/gpu_kernel_comprehensive_tests.rs

@@ -420,7 +420,7 @@ mod gpu_kernel_tests {
 
         // Check that center has highest value
         let center_val = phi_result[center];
-        assert!(center_val > 0.0, "Center value should be positive");
+        assert!(center_val < 0.0, "Center value should be negative (∇²φ = f, a positive source)");
 
         Ok(())
     }

crates/specialized/rtx-cfd/tests/gpu_kernel_tests.rs

@@ -256,7 +256,8 @@ mod cuda_tests {
 
         // Solution should be non-zero in the source region
         let center_idx = (ny / 2) * nx + (nx / 2);
-        assert!(result[center_idx] > 0.01);
+        // ∇²φ = f with zero boundaries: a positive source makes a NEGATIVE bump.
+        assert!(result[center_idx] < -0.01);
 
         // Boundaries should remain zero (Dirichlet BC)
         assert_relative_eq!(result[0], 0.0, epsilon = TEST_TOLERANCE);