A causal description of the universe that simultaneously respects Lorentzian geometry, quantum field theory, modular theory and holographic information bounds can be organized around small causal diamonds and their boundary algebras. In this work, a unified framework is constructed in which: enumerate \item The space--time background is encoded by a nested family of small causal diamonds (D_{p,r}) on a globally hyperbolic Lorentzian manifold. Their partial order and generalized entropy arrow def