BDPD³ — Governing the Signal, Not the Symptom: Leading-Indicator Regulation Prevents the Seneca Cascade in a Polycentric Commons

A polycentric commons in which one jurisdiction's industrial pollution flows downwind into its neighbours raises a governance question usually framed as which lever — a cap, a levy, a fine. Using the BDPD platform on a Bardi/Seneca three-variable engine (resource R, capital C, pollution P), we show the binding axis is not the lever but the trigger signal. Pollution is a lagging tail of the industrial boom: a regulator reacting to it is structurally too late, and the downwind victim is not saved. Arming the same cap on the capital stock — a leading, player-visible indicator — prevents the cascade AND rescues the emitter from its own Seneca cliff. A structural OFAT robustness sweep confirms the claims hold across the model's main parameters; the full win-win holds up to integration step dt ≤ 3.0.