SECTOR PAPER XX — ZERO‑PARAMETER PARITY‑ODD POLARIZATION INVARIANT Magnetar‑Domain Detection of the δI‑Substrate via Circular–Linear Mode Coupling.

Sector XX presents the zero‑parameter magnetar‑domain polarization invariant predicted by the Unified Information Substrate. The δI‑substrate generates a forced circular–linear mode‑coupling signature in extreme magnetic‑field regimes, producing a parity‑odd polarization rotation that cannot be reproduced by classical plasma physics, QED vacuum birefringence, or magnetospheric scattering models. This sector demonstrates that magnetar‑scale δI‑gradient amplification produces a stable, horizon‑proximal polarization invariant detectable in X‑ray and gamma‑ray polarization measurements. Sector XX establishes the first direct astrophysical detection pathway for δI‑substrate dynamics and forms the fifth component of the horizon‑collapse chain.