Path connectedness of multistationarity in a two-layer phosphorylation cascade with a shared phosphatase

Unrefereed candidate. A written proof of path connectedness of the full positive sixteen-dimensional rates-and-totals multistationarity region for a two-layer phosphorylation cascade with a shared phosphatase. An exact rational overlap certificate and a zero-to-negative ray address Conjecture 4.3 of Kaihnsa and Wang, arXiv:2607.25456v1. Includes exact algebraic checks and internal model editorial reports. No formal verification, authenticated external review, stability result, physiological validation or historical-first claim is made. Original prose/data CC0; code MIT; cited and supplied third-party files not redistributed.