Report Rg and I(0)

[mlund]

Add Rg / I(0) outputs, scoped by quantity:

• Geometric Rg (form-factor-weighted, from coordinates): always-on, no fit. For trajectories report mean ± spread (per frame; per molecule in multi-molecule runs).
• Guinier Rg + I(0) (low-q fit): single-particle only — from I(q) for single structures, from ⟨I(q)⟩ for single-molecule trajectories, and from P(q) in multi-molecule periodic runs. Do not fit Guinier to total I(q) when S_eff(q) ≠ 1 (inter-particle correlations distort low-q); suppress or flag it there.

Cheap; good validation cross-check (geometric vs Guinier Rg agreement guards against make-whole/PBC/units errors).