The AC Aperture: A Three-Stream Metric Normalization for the E1P Alpha Bridge

This paper develops the AC aperture programme within Energetic First Principles as a proposed route to the remaining metric datum in the E1P alpha bridge. The alpha-bridge derivation localizes the fine-structure constant problem to a single saturation parameter: the metric scale of the loading coordinate. The present paper reformulates that parameter as an aperture intrinsic to the AC phase. The central idea is that AC is not merely a closure operation, but a three-stream separation of one act into closure, spectra, and remainder. Closure corresponds to balanced passage, spectra to Active-excess radiation, and remainder to Connective-excess residue. This three-stream partition supplies the natural geometric setting for a non-projective metric normalization. The paper introduces a stream-simplex formalism for AC, defines the AC aperture as the angle separating transmissible closure from rejected or radiated excess, and shows how such an aperture would determine the saturation ceiling required by the alpha bridge. It then presents a theorem programme for deriving the aperture from internal E1P constraints: exhaustion of the AC act, ratio character of the three streams, conjugacy of Active and Connective excess, Ratio Within, and stability of clean three-stream separation. The result is a precise reduction of the remaining numerical problem. A complete derivation of the low-energy fine-structure constant from E1P requires an independent derivation of the AC aperture from three-stream geometry. Once that aperture is obtained, the alpha-bridge machinery converts it into the physical coupling value. This draft is therefore a structural companion to the E1P alpha-bridge paper. It does not claim a completed numerical derivation of the fine-structure constant. It identifies the next mathematical object to be derived: the unique geometric aperture selected by the closure, spectra, and remainder separation of AC.