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 definition of the locked observable f̂ via per-qubit POVM tomography + group symmetrization + singular-value-based singlet dimension estimation, mundane-source noise budget with pre-test gate (1/16), statistical-power requirement, Bayes-factor decision rule, and falsification criterion. Designed to be testable on current superconducting-transmon hardware (e.g. IBM Fez) where the d = 5 budget gate is not yet achievable. No retroactive parameter tuning, target relabelling, architecture reclassification, or modification of the f̂ definition is permitted under this lock. SHA-256 of pre-registration file: f7b99b1f70f4b5d28281da686443ccf7d1ebd36eaee252b121c526254da48b83Byte count: 16654Stage: 1b of 2 (protocol and prediction lock at d = 3 / m = 3, platform-agnostic core)Companion to Stage 1: DOI 10.5281/zenodo.20360004