Tests: Exact (Philip, 1960)

This test verifies the classic exact case where D(θ) = (1 - log(θ)) / 2 ⇒ θ(o) = exp(-o).
It uses the neural API frontx.neural.fit to match the reference curve and checks the absolute error.

How to run

From the repository root:

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