Fix divide-by-zero in aromaticity index calculation

[robertyoung3]

Summary

• _calc_ai() and _calc_ai_mod() in MolecularFormulaCalc divide ai_n / ai_d without guarding against ai_d == 0
• This raises ZeroDivisionError for molecular formulas where C - 0.5*O - N - S (or C - O - N - S for modified AI) equals zero
• Returns 0 when denominator is zero, consistent with the existing clamping to [0, 1]

Before

ai = ai_n / ai_d

After

ai = ai_n / ai_d if ai_d != 0 else 0

Applied to both _calc_ai() (line 563) and _calc_ai_mod() (line 604).

References

• Koch and Dittmar, 2006: https://doi.org/10.1002/rcm.2386
• Correction: https://doi.org/10.1002/rcm.7433

Test plan

• Verified both methods have the same pattern and same fix
• A denominator of zero means no aromatic carbon skeleton is possible, so AI = 0 is the correct semantic value