Breaking Ontological Equivalence via Horizon Agency: The Procrastination Gedankenexperiment and Automatic Golden-Branch Selection in Time-Symmetric Quantum Darwinism

In earlier works on Time-Symmetric Quantum Darwinism (TSQD), we demonstrated a strict four-way retrospective equivalence (Paper VII) between Single-History Retrocausality (the Two-State Vector Formalism, TSVF), All-at-Once Block Universe constraints, Everettian Many-Worlds measure concentration, and Relational Quantum Mechanics (RQM) cross-perspective links. For any observer conditioning purely on past empirical data (t <= t_now)—such as the 1 L_sun main-sequence stellar selection bound and the 60-degree Cosmic Microwave Background (CMB) angular correlation cutoff—these ontological interpretations yield identical transition probabilities via the Aharonov-Bergmann-Lebowitz (ABL) rule. In this paper, we introduce the Horizon Procrastination Gedankenexperiment and demonstrate that this four-way equivalence is decisively broken by future-directed epistemic agency. When a technological civilization discovers that macroscopic classicality requires saturating the asymptotic de Sitter horizon (I+) beyond the Page-curve redundancy threshold (f_A = 0.5) via a coherent stellar beacon, a single-history retrocausal ontology induces a pathological "immortality moral hazard." Because pre-trigger extinction would violate the post-selected boundary fixed point Phi* = S[Phi*], a single-history agent is perversely incentivized to delay firing the beacon indefinitely to preserve collective quantum immortality, generating an unstable causal-loop bilking paradox. Conversely, formulating the horizon boundary condition as an Everettian branch-selection projector Pi_cl(I+) acting on the universal wavefunction Psi resolves the pathology completely. Because unitary evolution explores the full dynamically accessible Hilbert space, the existence of at least one horizon-saturating future branch (the "Golden Branch," with positive measure mu(B_golden)) is guaranteed automatically, anchoring the shared 13.8-Gyr historical trunk without retrocausal fine-tuning. Furthermore, because sub-threshold branches fail to stabilize macroscopic pointer bases, observer self-location naturally tracks the classical Golden Branch while assigning direct ontological weight to branch-level agency.