Observable-Space Rank-1 Structure in a Viable Interacting Dark Sector

This record contains the final fixed version of Paper B, which characterizes the effective dimensionality of a viable interacting dark sector set in observable growth space. Main result (observable-space SVD): • We form a six-dimensional growth observable vector R(z) evaluated at z = (0.3, 0.5, 0.8, 1.0, 1.5, 2.0) across 76 viable configurations. • The leading SVD mode explains 99.945% of the total variance (PC2 = 0.055%, PC3+ < 10^{-3}%). • Bootstrap robustness (10,000 iterations; 70% subsampling): mean 99.944%, σ = 0.010%, minimum 99.880%, 2.5th percentile 99.921%. This work is a geometric/structural characterization (effective rank-1 in observable space) and is not intended as a model-selection claim versus ΛCDM. A companion work (Paper A) documents the underlying viable-band construction and baseline phenomenology.