Spinor-Valued Photocity Field: Torsion, Einstein–Cartan Emergence, and a Falsifiable Prediction from Magnetar X-ray Polarimetry

We present a classical effective spinor–tensor extension of the scalar photocity field theory (Dan 2026, DPME preprint, doi:10.5281/zenodo.20373859). The photocity field is promoted to a Dirac spinor in the spinor bundle over a pseudo-Riemannian spacetime. A covariant first-order (Palatini) action is constructed; variation yields four coupled field equations — the Spinor Photocity Master Equation (SPME). Einstein–Cartan torsion emerges from the spin-connection variation with no additional postulates. Singularity avoidance is enforced by the Quantex regulator F = (1 + ξ²)⁻¹, the minimal smooth positive function satisfying covariance, positivity, dimensional consistency, finite high-density saturation, and GR recovery. The SPME reduces to the scalar DPME, to Einstein–Cartan GR with a derived cosmological constant Λₑₒₒ, and to Newtonian gravity in appropriate limits. The framework is a classical effective theory below the Planck scale. A falsifiable, parameter-free prediction is derived: vacuum birefringence in magnetar magnetospheres is suppressed below the Euler–Heisenberg (QED) value by F(ϱᵇ) = (1 + ξ²ᵇ)⁻¹, with the correction scaling as B⁴ and completely independent of photon energy. This B⁴, energy-independent scaling is distinguishable from loop-quantum-gravity and doubly-special-relativity predictions (which scale as (E/Eₚ)² with n = 1 or 2) and is testable via systematic comparison across the ongoing IXPE magnetar survey.