BDPD⁴ — A Fully Grown Forest of Humbaba: A Card Game to Explore Commons Governance, and a Laboratory to Test It

We present the fifth installment of the BDPD (Be Different Play Differential) project: an extension of The Forest of Humbaba, the tabletop component of its dual-mode common-pool-resource laboratory, together with the experimental apparatus that tests it. The card game, previously limited to the single-commons extraction baseline of BDPD⁰, is extended through three additive modules — Governance, Polycentric, and Seneca — that project the computational platform's governance surfaces (pacts, cheap talk and graduated sanctions; nested worlds with treaties; leading-indicator regulation) onto physical components, each carrying a printed mapping back to the simulator. Alongside the game we describe a reproducible experimental apparatus — a single source-of-truth ledger, gate criteria defined per experiment, and persisted traces with offline invariants that assert what the engine did rather than what it printed. The two are exercised together in a worked study of graduated sanctioning under enforcement without disclosure, whose primary result is a null: making the sanction schedule known reached the players but did not change their extraction, and the one downstream effect resembling successful governance proves to be an accounting artifact rather than a behavioural response. Both deliverables are offered as reusable — the extended game as a design specification playable as written, the apparatus as a template for further analysis over existing data.