De Tempore Luminis Einstein's Forgotten Equation — $c$ is the Speed of Time, $E=mc^2$ is the Equation of Time

For 120 years, physics has interpreted $E=mc^2$ as a relation between matter and light \citep{Einstein1905}. We present the final consolidated manuscript of TER v4.0, De Tempore Luminis, where we prove that this interpretation is ontologically incomplete. We propose the Time-Light Equivalence Principle: $c$ is fundamentally the speed of time $v_T = 299,792,458$ m/s, not the speed of light. Light is time made visible at $v_T$. Mass is condensed time $M_T$, Energy is energy of time $E_T$. The master equation $E_T = M_T \cdot v_T^2 \equiv E=mc^2$ is identical numerically to Einstein's equation but reveals its true meaning: it is the equation of Time.