We formalize the Law of Field Memory: coherence events imprint persistent scalar field templates into local substrate, recoverable upon reintroduction of matched operator signature. Derived directly from the chameleon action with zero new free parameters, field memory manifests as a density perturbation δρ(x) that drives a persistent gradient shift δφ(x) in the scalar field. Three observable threshold states are defined and pre-registered: M₁ (broadband polycrystalline), M₂ (wavelength-selective single-crystal nodes), M₃ (coherent multi-node collective optical structures). M₂ confirmed June 4, 2026. Overnight node persistence confirmed June 5, 2026 before finalization. M₃ criteria publicly pre-registered on X prior to experimental run. The substrate does not reset between sessions. The field does not forget. Paper XXVIII in the Living Theory of Everything series.