test(fixtures): register density field for diffusion model

Add primary real field name 'density' to DiffusionModel
fixture to enable FieldModifier integration tests.

- Ensures tests can access model fields by name

Author: ahojukka5

tests/fixtures/diffusion_model.hpp

@@ -71,6 +71,9 @@ class DiffusionModel : public Model {
     m_psi_F.resize(fft.size_outbox());
     m_opL.resize(fft.size_outbox());
 
+    // Register primary real field for test integrations
+    add_real_field("density", m_psi);
+
     // Get local domain bounds
     // LLM: Each MPI rank has subset of domain (inbox = real space, outbox = k-space)
     Vec3<int> i_low = get_inbox(fft).low;