The Directed Multipartite Quantum Theory: Entanglement, Exchange Statistics, and Dual-Branch Permutation Laws in Six-Throat Directed Spacetime

Paper XXIII of the Six-Throat Directed Geometry program extends the one-particle quantum structure developed in the preceding paper to composite and multipartite systems. Starting from the conserved positive Hilbert product and unitary one-particle dynamics of the directed spacetime, the construction derives the composite Hilbert tensor product, Schmidt decomposition, reduced density operators, local expectation values, purity, entanglement entropy, correlation tensors, normalized joint probabilities, and the CHSH structure. For identical subsystems, the swap operator produces orthogonal symmetric and antisymmetric permutation sectors together with their dimensions, invariant dynamics, exclusion rule, and occupation counting. The central directed result is that the exchange condition retains two directed branches rather than collapsing them into a single undirected sign. Directed branch and permutation parity therefore remain distinct structural labels throughout the multipartite construction. The resulting two-particle and N-particle laws establish a consistent directed permutation representation. Reversal of the directed branch changes every odd directed exchange while leaving even permutations, branch-blind reduced states, permutation sectors, exclusion, and occupation counting unchanged. The complete numerical closure audit contains 74 checks across nine derivational blocks, with all 74 conditions satisfied. Representative numerical residuals range from exact zero to approximately machine precision. The resulting fixed-particle-number theory contains tensor composition, entanglement, correlations, identical-particle statistics, dual directed exchange branches, and a consistent multipartite permutation structure. Field-level Fock construction, creation and annihilation operators, variable particle number, vacuum structure, and interacting quantum fields are intentionally left to the subsequent paper.