The resummation was previously applied to all energy points when the
flag was set, causing resummed and standard modes to differ everywhere.
Per spec, it should only replace C(beta)*ln(W0-W) with the resummed
power-law form when (W0-W) < delta_cut (~1e-3).
Adds delta_cut parameter (default 1e-3) and corrects the resummation
switch test to verify identical results far from endpoint.
- Add Z (nuclear charge) and A (mass number) parameters to RadiativeCorrection
- Implement O(Z*alpha^2) finite nuclear size correction per Sirlin 1987:
- Energy-dependent part: ln(m_p) - (5/12)*ln(2W) + 43/18
- Nuclear-structure-dependent part: Delta_F(Z,A) from modified Gaussian model
- Add small-beta Taylor expansion for numerical stability at threshold (beta < 1e-6)
- Extract _safe_beta() and _safe_tanh_inv_beta_over_beta() helper methods
- Remove unused log_wright_bessel import
- Add 14 new tests: Z-dependence, numerical stability, A parameter coverage
- Update docs/06-radiative-corrections.md with new API usage examples
All 70 tests pass. 14 new tests in 3 new test classes.
unzhakov