  Given a complex reductive group G, Borel subgroup B, and topological surface
S with boundary dS, we study the "Betti spectral category" DCoh_N(Loc_G(S, dS))
of coherent sheaves with nilpotent singular support on the character stack of
G-local systems on S with B-reductions along dS. Modifications along the
components of dS endow this category with commuting actions of the affine Hecke
category H_G in its realization as coherent sheaves on the Steinberg stack. We
prove a "spectral Verlinde formula" identifying the result of gluing two
boundary components with the Hochschild homology of the corresponding
H_G-bimodule structure. The equivalence is compatible with Wilson line
operators (the action of Perf(Loc_G(S)) realized by Hecke modifications at
points) as well as Verlinde loop operators (the action of the center of H_G
realized by Hecke modifications along closed loops). The result reduces the
calculation of such "Betti spectral categories" to the case of disks,
cylinders, pairs of pants, and the M"obius band.
