The Interior Engine of Spacetime: A First‑Principle Model of Compact Cores. A Finite, Predictive, and Falsifiable Interior Geometry.

Astrophysical observations show that galactic centers host stable, highly organized structures—collimated jets, coherent disks, long‑lived precession patterns, and ordered inflow and outflow channels. These features require a finite, rotating interior capable of storing angular momentum, redirecting motion, and shaping the surrounding spacetime. Classical general relativity provides no such interior: the Schwarzschild solution ends in a singularity, while the Kerr interior is unstable and contains unphysical pathologies. A realistic galactic engine therefore demands a nonsingular interior geometry with well‑defined structure and internal degrees of freedom. This paper develops a complete geometric model of such an interior. The construction is fully classical and does not invoke exotic matter or modifications of gravity. Instead, the interior is treated as a structured region of organized curvature, torsion, and circulation. Its internal state is encoded in seven dimensionless kernel variables—curvature pull, torsion pull, toroidal and poloidal spin, drift, matter exhaust, and temporal exhaust—which represent the natural geometric channels available in a stationary, axisymmetric core. These variables couple to the metric through an effective stress‑energy tensor derived entirely from geometric motions. We construct a self‑consistent interior metric with torsion and a preferred direction field, derive the Hamiltonian, momentum, and torsion constraints, and obtain the coupled system governing the evolution of the metric and kernel variables. The resulting interior is finite, nonsingular, and dynamically organized. We analyze its geodesic structure, identify toroidal valleys, trapping surfaces, and axial channels, and establish the stability of circular, vertical, and axial orbits. These features determine how matter and photons move within the engine and how the interior influences the surrounding spacetime. Finally, we show how the interior geometry produces the exterior geometric tail developed in Paper 1 and demonstrate that the interior solution matches smoothly onto the corrected exterior metric. The model predicts rotation‑curve behavior, lensing corrections, vertical confinement, jet alignment, disk warps, and transparency‑shell features, providing a geometric foundation for the observed organization of galactic centers.