Exact Analytical Phase Transitions, Horizon Bistability, and Thermodynamic State-Space Representation of Regular Hayward Black Holes
We establish exact analytical thresholds and present a unified thermodynamic state-space representation for the regular Hayward black hole, resolving the full phase structure without reliance on numerical approximations. By evaluating the Hawking temperature, Helmholtz free energy, and heat capacity against the classical Schwarzschild baseline, we derive the exact geometric watershed for the zero-temperature extremal remnant at $r_h = \sqrt3l$, along with the exact Davies critical transition at $r_h = 3l$. Within the intermediate regime $\sqrt{3}l < r_h < 3l$, we uncover a distinct horizon bistability where two distinct horizon radii share identical free energy and temperature profiles. We demonstrate that the Davies singularity acts as a precise thermodynamic divide, separating a locally stable ($C_V > 0$) quantum Small Black Hole branch from an unstable ($C_V < 0$) Large Black Hole branch. Furthermore, we evaluate the exact integrated non-area law entropy at the critical turning point, yielding $S_{turn} = \pi l^2 \left[ \frac{63}{8} + 2\ln(8l^2) \right]$. Finally, we introduce a compact, 3-parameter thermodynamic state-space diagnostic that tracks the continuous evolution of regular black holes from classical thermal evaporation to cold remnant lock, offering a quantitative framework for quantum-gravity phenomenology.