We present a field-theoretic framework establishing the Photo-Inertial Effect (PIE), wherein the effective inertial mass of a localized matter wave is dynamically tuned via resonant electromagnetic gauge-phase coupling. Building upon the self-contained framework of covariant quantum clock fields, we model massive particles as extended quantum clocks cycling at their characteristic Compton frequency. We provide physical justification for transforming the canonical Hamiltonian energy partition into an active conservation law. Crucially, we resolve the central theoretical question of why resonant matter waves preferentially undergo coherent mass translation over standard quantum electrodynamic (QED) scattering. We prove that local gauge invariance renders linear phase-gradient coupling mathematically mandatory. While standard quantum electrodynamics erases this interaction under point-particle scattering approximations, Compton resonance converts it into a global parametric drive. Because phase gradients and spatial position are canonically dual, preserving fiber bundle integrability compels the wavepacket core to translate coherently rather than undergo decohering scattering. This suppresses effective inertial resistance toward zero and establishes a testable, parameter-free framework.