The Canon 4D‑Loop Synthesis: The Universe is a 4D Klein Block Starting from the Identity Constraint (I → A = A) and three interlocking foundational principles—the Necessity of Integrity (NoI), the Necessity of Sufficient Termination (NST), and the Single‑Reality Standard (SRS)—this paper derives the mandatory global structure of the universe. Logic and the observer are co‑sovereign: logic provides the structure; the observer—the Read‑Head—provides the verification without which the loop cannot close. The unique geometry satisfying all constraints is a compact, boundaryless, non‑orientable 4‑Dimensional Klein Block. This geometry yields three topologically mandated, falsifiable empirical signatures (Klein‑Parity Rings in the CMB, a modified primordial tensor spectrum from half‑spacing SGWB harmonics, and a Z₂ neutrino partition in the Cosmic Neutrino Background) and provides conceptual resolutions for fifteen major cosmological paradoxes. The deductive chain establishes that the whole of existence must be: - Mandatory – absolute nothingness is logically impossible; existence is the default state of reality.- Finite – actual physical infinity is incompatible with a completed, determinate identity.- Static – all spatial and temporal coordinates coexist; reality is a 4‑dimensional block.- Self‑contained with zero total energy – the positive energy of matter and the negative potential energy of gravity cancel exactly (H_total ≡ 0).- Temporally closed – the Heat Death and the Big Bang are the same coordinate, identified through conformal scale invariance.- Globally non‑orientable – forced primarily by the geometric impossibility of a monotonic entropy gradient on a closed orientable loop (Theorem 2), and independently confirmed by the NST+SM selection principle. What is new in Version 15 V15 is the historical definitive edition of the framework, freezing the full deductive architecture, empirical matrix, and particle‑physics roadmap. All mathematical appendices, the audit trail, and the original conceptual machinery (NoI/NST/SRS) are preserved. Key features of V15: - Single‑Reality Standard (SRS) formally introduced (§1.2.1). A geometry that cannot host a Read‑Head is single‑reality inadequate.- Three axioms unified (§1.2). NoI, NST, and SRS are the three faces of a single demand: determinate, minimal, and self‑verifying.- The Hidden Nothingness Assumption exposed (§1.2.2). Intuitive objections based on external voids and process‑loops are systematically refuted.- Appendix B elevated. The Read‑Head is a structural necessity of the deductive chain, not a speculative coda.- Empirical signatures honestly characterised. Signature 1 (qualitative, pipeline‑ready protocol in Appendix D), Signature 2 (semi‑quantitative, pending full simulation), Signature 3 (motivated hypothesis, 50–56% capture‑rate interval, falsifiable by PTOLEMY).- Exact analytic topological fluid derived (Appendix A.6) — the Hubble tension pipeline is sealed.- The framework is now fully transparent about what has been proved and what remains conjectural. Separation of the irreducible core The final, irreducible core of the framework—from which all optional premises (NoI/NST) have been eliminated and which leaves no escape for the sceptic—is published as a separate, self‑contained paper: "The Necessary Geometry of the Maximal Whole" (10.5281/zenodo.21247144) That paper proves the universe is necessarily a 4‑Dimensional Klein Block from only the Law of Physical Determinacy (LPD) plus the existence of the observer and the observed entropy asymmetry. The present V15 edition remains the comprehensive reference for all detailed derivations, appendices, and the full empirical programme. Forthcoming data from JUNO, LiteBIRD, the Simons Observatory, CMB‑S4, and PTOLEMY will provide definitive tests of the predicted signatures. www.necessaryuniverse.com