The Interior Observer Cosmological Framework — Paper 21: The AC1 Derivation, the SU(2) Geometric Weight, and the Radiation Response Problem

Paper 21 of the Interior Observer (IO) Cosmological Framework. Beginning from two premises — (1) the observable universe exists inside a Schwarzschild black hole, and (2) the physics inside the horizon is the same as outside — this paper derives AC1 (the acoustic angular scale Jacobian) from the existing Papers 15–20 stack, upgrading Paper 20's Theorem 20. 2 to DERIVED/THEOREM status within the reduced scalar/longitudinal acoustic sector. The zero-parameter prediction θₚred = 0. 599° (Planck: 0. 597°, residual 0. 429%). Part II investigates the structural origin of the D/H radiation deficit, discovering the SU (2) Geometric Weight (Zₚunc = 1. 4221), proving the Optical Filtration Theorem (direct reduced RT/BY optical readout of θ is blind to noncentral SU (2) puncture data), and deriving the Absolute Local Casimir Load Fₐbs = 0. 36879 matching the D/H target to 0. 066% precision (STRUCTURALLY SUPPORTED CONSTRUCTION). Nineteen routes (eighteen Pᵣesp plus freeze-in) are tested and killed. The surviving target is a non-separable, channel-dependent radiation response operator. https: //dfife. github. io/index. html v1. 4 (April 2026): Appendix replaced with Paper 20 v1. 5 clean + Paper 21 results. AC1 closed. Open/Closed tracking. v1. 3 (April 2026): Full Schur branch appendix correction. All inherited appendix steps (29, 56, 58, 60, 107, 122) and open-problem Item 16 updated to Schur definitive branch (H₀ = 68. 91, Paper 29). Paper 19 intermediate supersession notes updated. Bare branch (H₀ = 61. 06) references in §§7, 13, 14 marked SUPERSEDED (Paper 29). Title page standardized. All body theorems (AC1 derivation, SU (2) geometric weight, sky-slot inheritance, optical filtration, Fₐbs) are branch-independent. Version 1. 2 includes cycloid parameterization correction, clean Appendix A catalog (Steps 1–207) with Papers 18–20 status updates applied, and Wolfram/Gemini hostile review passes.