Tests: frontx.solve

This suite exercises the forward solver along three fronts:

1. Exact case (Philip, 1960): with D(θ) = (1 − log(θ)) / 2, the solution is θ(o) = exp(-o).
2. External reference comparison (fronts): for LETd, LETxs, VanGenuchten, and Brooks–Corey models it checks:
3. agreement of D, dD/dθ, and d²D/dθ² with fronts.D,
4. agreement between frontx.solve and fronts.solve (abs ≤ 5e-2).
5. Unsolvable cases: the solver must raise or flag failure (result != successful) under impossible conditions.

How to run

From the project root:

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