Born Complex Structure from Contexts:Complex Structure from Measurement Contexts in a Reconstruction of the Born Rule: The Phase Commutant, Selection of 𝑖, and the Origin of Unitarity

We reconstruct complex quantum kinematics from a finite-dimensional real amplitude space equipped with projective measurement contexts and a positive mass geometry. Contextual completeness makes the generated algebra irreducible, so its neutral commutant has real, complex, or quaternionic scalar type. A positive-dimensional abelian group of neutral continuous phases selects exactly the complex branch. Normalizing its central generator yields a common operator J2 = −I, unique up to J ↦ −J, while the double commutant recovers the full algebra of complex-linear operators. The identity component of the orthogonal normalizer acts unitarily, and normalizer elements reversing the sign of J act antiunitarily. A commutator Laplacian identifies the neutral commutant and provides spectral-gap estimates for approximate neutrality and stable phase reconstruction. A three-context qubit example gives an explicit finite realization.