Virasoro zero-mode shift (${\mathcal{L}}_m=L_m-a{\delta }_{m,0}$)

Shifting the zero mode changes the algebra's central representative:The linear term follows from replacing $L_{m+n}$ by ${\mathcal{L}}_{m+n}+a{\delta }_{m+n,0}$. It changes the zero-mode convention, not the cubic coefficient or central charge. This distinction separates the intercept from the Virasoro central extension.