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>

tests/conftest.py

@@ -0,0 +1,24 @@
+import sys
+import os
+sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
+
+import pytest
+import torch
+
+
+@pytest.fixture(scope="session")
+def device():
+    from engine import _get_device
+    return _get_device()
+
+
+@pytest.fixture
+def small_data():
+    from engine import prepare_data
+    return prepare_data(N_f=200, N_ic=50, N_bc=50)
+
+
+@pytest.fixture
+def pinn_model(device):
+    from model import HeatPINN
+    return HeatPINN().to(device)