This is a polyfill for the TC39 Signals proposal, implementing a first-class JavaScript data type for unidirectional data flow through state cells and derived computations. It provides `Signal.State` for creating reactive state cells and `Signal.Computed` for defining computations that automatically update when their dependencies change. The polyfill is intended for framework and library authors to experiment with the Signals proposal and build their own reactive systems, not for production use as the proposal is still evolving.