PIVI — Phase-Instability Velocity Index: A Testable EEG-Derived Candidate Marker for Early Forecasting of Cognitive Fatigue

This preprint introduces the Phase-Instability Velocity Index (PIVI), a proposed EEG-derived research-use candidate marker designed to quantify the temporal velocity of phase-connectivity destabilization during sustained cognitive load. The manuscript presents the theoretical rationale, operational definition, falsifiable hypotheses, claim boundaries, public-data evaluation criteria, and validation gates for testing whether PIVI can forecast cognitive fatigue onset before overt behavioral decline or subjective fatigue escalation. PIVI is presented as a research-use candidate index. It is not presented as a validated clinical biomarker, diagnostic tool, treatment, preventive intervention, or deployment-ready system. The article defines a conservative validation pathway requiring signal verification, analytical validation, prospective prediction testing, independent replication, artifact and confounder control, and careful claim governance before any stronger translational or clinical language would be justified. The contribution of this record is a public scientific formulation, not a claim of completed empirical validation.