This paper is the third in a series developing a geometric and operator-theoretic framework for the Riemann Hypothesis and related Millennium Problems. The first paper established the geometric foundation: Dirichlet partial sums embedded on S² via stereographic projection, with log-inertial dynamics reproducing the statistical structure of the Prime Number Theorem. The second paper developed the convergence and compactification structure: canonical prime measures on S², spectral dichotomy of the associated integral operator, and Lyapunov stability toward a unique attractor on the critical line. The present work completes the program by demonstrating that convergence and completeness, viewed from multiple independent perspectives, realise a single conservation principle — and that this principle simultaneously governs three Millennium Problems under explicit conditional assumptions. The central observation is geometric: the sphere S² is a closed manifold. Once the prime bundle system is embedded on S² and the BF bound saturation condition m²_{AdS₂} = −1/4 is adopted as the structural condition, Cylinder Completeness — previously assumed as an axiom — follows from the first law of thermodynamics: energy conservation across the isolated AdS₂×S² system forces INS=IYM, and σ = 1/2 is the unique value at which the cylinder neither leaks nor explodes (Theorem 8.4). The interior and exterior of the system thereby become two faces of a single indivisible object — not by assumption, but by energy conservation. Navier–Stokes regularity and the Yang–Mills mass gap emerge as the two sides of this object — not separate problems, but the same conservation law expressed in different physical languages. When σ ≠ 1/2, energy transfers asymmetrically between the two faces: for σ > 1/2 the effective viscosity acquires an imaginary part and energy oscillates without dissipating; for σ < 1/2 the prime measure diverges and energy explodes. In both cases information leaks out of the cylinder and all bulk-boundary correspondence is destroyed. σ = 1/2 is the unique equilibrium. The Riemann Hypothesis then appears not as a statement about zeros of a complex function, but as a thermodynamic equilibrium condition: the unique normalisation σ = 1/2 at which the intertwining operator T_{1/2} mediates between the two boundary faces without information loss. It is the sole critical point at which the system conserves energy symmetrically across both faces simultaneously. Five algebraically independent paths (Paths A–E), Prime Bundle Irreducibility, and Covering Map Rigidity all converge to the same conclusion from different directions — algebraic, entropic, geometric, operator-theoretic, and analytic. This convergence is not coincidental. It reflects the fact that the closed manifold admits only one consistent internal standard compatible with energy conservation: the critical line. The unified principle proposed here is conditional but structurally complete: *Under the small-data condition ∥Z0∥H2<4.91/α\|Z_0\|_{H^2} < 4.91/\alpha ∥Z0∥H2<4.91/α and the first law of thermodynamics applied to the isolated AdS₂×S² system, a closed manifold with two observable faces and a well-defined intertwining structure must be calibrated at σ = 1/2. The three Millennium Problems are three ways of observing the same conservation law.*