test: aggiunge suite completa — unit, integration ed e2e (42 test)

- pytest.ini: configura testpaths, marker slow, output verboso
- tests/conftest.py: fixture condivise (device, small_data, pinn_model)
- tests/test_config.py: sanità parametri fisici e numerici, CFL, _pde_scale
- tests/test_model.py: HeatPINN.forward e heat_pinn_loss (shape, finiti,
  zero-weight analytici per IC e BC, scaling dei pesi)
- tests/test_engine_data.py: set_seed, _get_device, prepare_data
  (shape, bounds, device consistency, determinismo)
- tests/test_integration_pinn.py: pipeline dati→modello→loss→backward
- tests/test_e2e.py: FDM completo, visualizer FDM/PINN con tmp_path,
  training breve (2 test @slow)
- requirements.txt: aggiunge pytest>=7.0.0

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

pytest.ini

@@ -0,0 +1,5 @@
+[pytest]
+testpaths = tests
+addopts   = -v --tb=short
+markers =
+    slow: test lenti (training completo) — escludi con -m "not slow"