What if the Schwarzschild Sign Change is Physical? From Geometry to Physics in a Fully Complexified (3+1) Spacetime

The sign change of coordinates at the Schwarzschild horizon is usually treated as a coordinate artefact. This paper treats it as a physical process and follows the consequences. The resulting spacetime is (3+1)C: three complex spatial coordinates zk = xk +iwk (expanded real parts, com pact imaginary parts on S^3) and one complex time τ = t+iσ (macroscopic real part, compact Euclidean Hawking circle). From the Kähler structure of this spacetime and a single geometric parameter — the Berger deformation ε = 1.01027, fixed by the electron mass — all of fundamental physics follows without additional postulates. Part II derives, before any particle content is specified: the Heisenberg uncertainty relation (from the Kähler form ω = dx ∧ dw), special relativity (as holomorphic symmetry of τ = t + iσ), the dispersion relation E2 =p^2c^2 + m_0^2 c^4 (as the Pythagorean theorem of the ¯∂-norm), inertia and E = mc^2 (as Cauchy–Riemann mismatch resistance), confinement energy (as the temporal dual of the same Cauchy–Riemann mismatch), the equivalence principle mi = mg (as a det/tr identity), Einstein’s field equations (from Lovelock uniqueness), the fine-structure constant (from the Sasaki contact structure), and Bose–Einstein condensation and superconductivity (from compact-state coherence under the holomorphic Lorentz boost).Part III derives the complete particle spectrum: the gauge group SU(3) × SU(2) × U(1), the charge, colour, and chirality structure of the Standard Model, three fermion generations (topological; fourth excluded), matter/antimatter distinction, all fermion and boson masses, the electroweak mixing parameters, the CKM matrix (all 9 elements within 4,5%, including the CP-violating phase), the proton mass, and a complete dark-matter sector (mass spectrum, abundance, vanishing direct-detection cross section — all from the horizon flip, no new fields). In total, ∼43 quantitative observables are reproduced with a maximum deviation of 6% for geometric quantities and 1.3% for masses; downstream quantities (lifetimes, couplings, nuclear binding) agree to ≤3% after propagating framework-internal corrections — a self-consistency test that numerology cannot pass. Three independent determinations of ε (from me, mH, and MP) agree to 130 ppm. The framework makes falsifiable predictions: a mass-ratio-independent remnant spin a∗(q) ≈ 0.69 for binary black-hole mergers (testable with LIGO O4/O5), r = 0 exactly (testable with CMB-S4), and a neutrino luminosity gap in core-collapse supernovae (retro-consistent with SN1987A to 3%). The Kähler structure is not an addition to the Standard Model — it is its geometric origin. The Standard Model is the R3,1-projection of a (3+1)C geometry.