Two Parameter Deformation of Embedding Class-I Compact Stars in Linear $f(Q)$ Gravity

Recent multi-messenger observations, including gravitational wave detections of compact objects in the neutron star-black hole mass-gap region and precise measurements of high-mass pulsars, motivate mechanisms capable of enlarging the stellar mass window without arbitrarily stiffening the equation of state (EOS) toward the causal limit. In linear $f(Q)$ gravity of the form $f(Q)=\beta_1 Q+\beta_2$, the theory is dynamically equivalent to General Relativity at the geometric level and modifies stellar structure solely through a uniform rescaling of the matter sector governed by $\beta_1$. Consequently, linear $f(Q)$ alone does not introduce new geometric families of stellar solutions or alter classical compactness bounds. To overcome this structural limitation, we incorporate gravitational decoupling within an embedding class-I (Karmarkar) Vaidya-Tikekar configuration in linear $f(Q)$ gravity. While similar VT-based decoupling constructions exist in GR, the present framework introduces a controlled two-parameter deformation characterized by $(\epsilon,\beta_1)$: the decoupling parameter $\epsilon$ governs geometric deformation and EOS stiffness, whereas $\beta_1$ independently rescales the matter sector without altering the metric structure. This separation permits a direct comparison between GR and linear $f(Q)$ gravity at fixed geometric deformation, thereby isolating pure coupling-driven mass enhancement. We determine the admissible parameter domain from regularity, matching, causality and compactness requirements and derive an analytic compactness bound for the decoupled embedding class-I configuration. The combined action of $\epsilon$ and $\beta_1$ enlarges the accessible stellar mass window while preserving physical acceptability, allowing configurations compatible with recent high-mass pulsars and mass-gap candidates without exceeding causal limits.