This work constitutes SPT v1.3 — Part V and establishes the empirical observational baseline for the interstellar object 3I/ATLAS within the Structural Flux Paradigm. Radio monitoring conducted on December 21, 2025 resulted in a confirmed non-detection across standard GHz bands. Rather than indicating dynamical inactivity, this null result is interpreted as evidence of a laminar substrate coupling regime characterized by high coupling efficiency (η → 1) and low structural impedance (Zₕ). Within Structural Plenitude Theory (SPT), residual radiative dissipation (ℛ) emerges only when coupling efficiency is reduced by structural excitation of the vacuum substrate (Pleroma). The December 2025 silence therefore defines the calibrated control state against which subsequent emissions, expected during the Jovian approach in early 2026, can be quantitatively evaluated. This document provides the zero-point reference for predictive tests of non-gravitational dynamics and substrate-mediated radiative anomalies developed in later parts of the frozen SPT v1.3 release. Each part of v1.3 is assigned an independent DOI for traceability and version integrity.