Function baseline_reference_energy_active 

pub fn baseline_reference_energy_active(
    energies: &[f64],
    active: Option<&[bool]>,
) -> f64

Reference energy for the baseline log basis, computed over the active fit window only (issue #648).

The full grid extends far beyond the resonances of interest (a VENUS Ta grid spans 4.5 eV–2.3 MeV while the fit window is 8–45 eV). Centering the ln(E/E_ref) basis at the full-grid midpoint (≈3211 eV) instead of the active-window midpoint (≈19 eV) pushes every active bin to a large negative z, so the 1, z, z² columns stop being near-orthogonal and the baseline silently absorbs Doppler broadening — the fitted temperature runs away with warnings = []. Restricting the midpoint to the bins that actually enter the cost (the active mask) restores it.

active is the per-bin mask from crate::active_mask::build_active_mask; None (no fit_energy_range) is identical to baseline_reference_energy over the whole grid. A mask selecting no bins falls back to the full grid rather than returning NaN (the pipeline’s active-bin-count gate rejects an empty window upstream, so this branch is defensive only).