UCT No-New-Constants Prediction 2 (Stage 1b): Three-Level / Three-Qubit Quantum Readout Floor (Pre-Registration)

Stage 1b pre-registration of a no-new-constants forward prediction derived from the UCT compression-interface theorem at d = 3 / m = 3. Companion deposit to Stage 1 (DOI 10.5281/zenodo.20360004); does not modify Stage 1. Predicts the structured readout floor f_U(3) = 8/9 ≈ 0.8889 for U(3)-covariant readout and f_{Z_2^3} = 7/8 = 0.8750 for sign-channel readout (gap 1/72 ≈ 1.4%). Contains: theoretical input, prediction and discriminator, five-criterion architecture classifier, precise operational