Phenomenology of Leptophilic Gauge Interactions at Future $e^+e^-$ Colliders

We present an observable-driven study of leptophilic gauge interactions mediated by a neutral vector boson $Z_\ell$ at future $e^+e^-$ colliders. Focusing on the process $e^+e^- \to \mu^+\mu^-$, we investigate the sensitivity of angular observables to off-shell leptophilic interactions in regimes where interference effects dominate over resonant rate enhancements. The analysis is centered on the forward-backward asymmetry, which provides a robust probe of interference-induced angular deformations and retains sensitivity well beyond the resonance region. To characterize the relative angular response independently of the overall rate modification, we additionally introduce the interference-normalized quantity $\mathcal{I}_{\rm FB}$ as a diagnostic observable. Realistic collider effects, including initial-state radiation, beamstrahlung, and finite energy resolution, are consistently incorporated. Using benchmark configurations for FCC-ee, CEPC, ILC, and CLIC, we derive projected sensitivities in the $(M_{Z_\ell}, g_\ell)$ plane. The results demonstrate the complementarity between high-luminosity circular colliders and high-energy linear colliders, reflecting the interplay between precision measurements and the energy enhancement of interference effects. Overall, the present framework provides an experimentally grounded approach for probing leptophilic gauge interactions through interference-sensitive angular observables at future lepton colliders.