Horizon Branch Partitioning and the Fermi Paradox: Mutual Orthogonality of Page-Curve Projectors in Everettian Time-Symmetric Quantum Darwinism

In earlier works on Time-Symmetric Quantum Darwinism (TSQD), we showed that the emergence of macroscopic classicality across a cosmological causal patch requires an active technological civilization to saturate a fraction exceeding 50% (f_A above 1/2) of the asymptotic de Sitter event horizon (I+) via a phase-coherent 1 L_sun stellar beacon. Within a single-history retrocausal ontology, this Page-curve bottleneck resolves the Fermi Paradox via competitive preemption: only one civilization can monopolize the horizon's entanglement capacity, raising the philosophical objection of either extreme evolutionary coincidence or "retroactive erasure" of earlier alien civilizations. In this paper, building on our recent demonstration (doi:10.5281/zenodo.24054298) that future-directed agency selects the Everettian Many-Worlds formulation of TSQD over single-history post-selection, we reformulate and strengthen the TSQD resolution of the Fermi Paradox via the Horizon Branch-Partitioning Theorem. We prove that for any set of potential technological civilizations {C_i} within a shared causal patch of the universal wavefunction, the strict Page-curve inequality f_A(i) + f_A(j) <= 1 enforces exact mutual orthogonality between their respective horizon classicality projectors: Pi_i * Pi_j = 0 for all i != j. Consequently, asymptotic decoherence partitions the Everettian multiverse into mutually orthogonal classical branches, each supporting at most one horizon-saturating civilization. No alien civilization is retroactively erased, nor is biological evolution extraordinarily rare; rather, every civilization that achieves the horizon threshold necessarily self-locates within a classical branch whose past light cone is devoid of prior horizon-saturating technosignatures.