This work introduces a unified state-space framework for analyzing stability and performance in magnetically confined fusion plasmas. Instead of treating individual operational limits independently, the formulation interprets tokamak plasmas as systems evolving within a multidimensional parameter space bounded by stability constraints and thermonuclear performance requirements. A dimensionless constraint-density scalar is introduced to represent the combined influence of multiple stability limits, including pressure-driven magnetohydrodynamic instabilities, density limits, current profile effects, and radiative losses. In normalized constraint space, this scalar defines a geometric collapse boundary separating stable plasma configurations from disruption-prone states. Fusion performance is represented through a dimensionless fusion viability parameter derived from the Lawson triple product. Together, these quantities define the fusion persistence window, the region of parameter space in which plasma configurations remain both stable and capable of producing net fusion power. Within this formulation: Plasma discharges correspond to trajectories in multidimensional parameter space. Stability limits appear as a boundary surface in constraint space. Disruptions correspond to trajectories approaching or crossing the collapse boundary. Viable fusion operation requires maintaining trajectories inside the intersection of stability and performance regions. The framework provides a geometric interpretation of plasma operational limits and suggests a physics-motivated scalar indicator of disruption proximity. The formulation also produces testable predictions regarding disruption statistics, plasma trajectory structure, and optimal reactor operation regimes. This work is conceptual and intended as a structural framework for interpreting plasma stability and disruption behavior in tokamak systems. Future work may evaluate the constraint-density scalar using experimental databases from fusion devices such as JET, DIII-D, EAST, and ITER.