A complete open hardware disclosure of a modular thermal flywheel stack and gradient governance system for data center thermal management. The system operates across three scales — micro (rack-level phase change cartridges), meso (building-level thermal mass and heat recovery), and macro (campus-level seasonal storage and absorption chilling) — using a material-agnostic specification that allows any thermally appropriate material to fill each functional role. Gradient governance replaces active control with physical architectures that enforce thermal flow direction, magnitude, and timing through geometry and material properties alone. All architectures released under CC BY 4.0. The DOI timestamp constitutes permanent prior art. No patent is filed or intended.