Invariant Magnitude from Phase-Bearing Summaries: The Quadratic Necessity

Born-like squared magnitude is forced by stable composition, not assumed from physics. This paper derives a Born-like squared-magnitude rule as a representational necessity in a pre-physical program. Building on prior results that force (1) plural incompatible stabilization criteria, (2) route dependence with cancellation-capable summaries (phase-like structure), and (3) the need for a stable nonnegative "yield" map, we formalize a yield function rho from summaries to nonnegative real numbers under minimal requirements: phase invariance, additivity over exclusive outcomes, multiplicativity under independent composition, and symmetry under balanced regrouping (no preferred exclusive coordinates). Under mild regularity, multiplicativity restricts rho to a power-law family, and regrouping invariance uniquely selects the quadratic case. The result is a squared-magnitude extraction rule obtained without assuming Hilbert spaces, wavefunctions, inner products, dynamics, hbar, or measurement postulates. The paper concludes by identifying the next forced pressures toward complex realization and norm-preserving (unitary-like) evolution.