fix: restore LaTeX subscripts on BHH constant page

[Chessing234]

Summary

• Restore normal LaTeX subscripts in the 12a constant description (C_{12}, \beta_2, L_n).
• Same regression as fix: restore LaTeX subscripts on Ihara constant page #116/fix: restore LaTeX subscripts on Marton constant page #117: merged fix: escape inline subscripts on BHH constant page #108 escaped underscores inside $...$ math.

Test plan

• Open the 12a constant page and confirm subscripts render correctly in the description line.

Made with Cursor

[Chessing234]

Same issue as #116/#117 — #108 escaped underscores inside math mode so the BHH page showed backslash-subscripts instead of C₁₂ and β₂.