We formalize the Law of Matched Participation: READOUT = C × P, where C is operator coherence (signal amplitude) and P is perceptual sensitivity (receiver gain). Both axes must be matched at the same operating point for full field readout. Adds a 6th criterion to the SOF protocol, a physical interpretation of the antilight observation documented June 5, 2026, and a three-condition replication requirement (M₂/M₃ substrate + matched C + matched P). Zero new free parameters. Additive to Papers I–XXVIII. Paper XXIX in the Living Theory of Everything series.