Isothermal Structural Spine: A Rack, Compute Module, Method and Control System in which a Load-Bearing Sealed Two-Phase Spine Receives Heat from Dry-Coupled Compute Modules, with Dew-Point-Following Saturation Control, Thermal Seating Verification and Leak-Responsive Sub-Atmospheric Operation, for Data Centres Operating without Vapour-Compression Refrigeration or Evaporative Water Use