Tests: Grenoble Sand (synthetic → neural fit)

This test generates synthetic data with a VanGenuchten model (Grenoble-sand parameters) and checks that fitting with frontx.neural.fit reproduces the reference curve.

• Reference: VanGenuchten(Ks, alpha, m, theta_range=(0, θs))
• Fit: VanGenuchten(Ks=Param, m=Param, theta_range=(0, θs))
• Check: |θ_pred(o) − θ_ref(o)| ≤ 1e-3 for o ∈ [0, oi_ref]

How to run

```bash pytest -q tests/test_grenoblesand.py