Observer-Covariant Quantum Channels on Growing Causal Diamonds

We formulate quantum evolution on growing causal domains as a family of quantum channels between generally non-isomorphic state spaces. Unitarity is replaced locally by approximate isometric evolution, measured with an energy-constrained diamond norm, while observer covariance is imposed through consistency, refinement, and cocycle conditions on causal overlaps. Applied to an evaporating black hole, the formalism yields a discrete isometric model in which information is transferred from the black-hole sector to the radiation sector without requiring a fixed global Hilbert-space factorization. A random-isometry approximation reproduces the Page transition and shows that a dynamically populated complementary sector produces a calculable endpoint defect in the Page curve. Information--disturbance bounds further imply that exact exterior unitarity forces the complementary channel to be input independent, while approximate unitarity quantitatively bounds structural leakage. The resulting framework interprets unitarity as an emergent property of observer-consistent causal descriptions rather than as a fundamental identification of state spaces at different times.