World construction

Create a World from Variable, Parameter, and ContinuousLaw values. State variables need exactly one law. Controls may appear in expressions but never receive a state-transition law. Parameters share the identifier namespace with variables, so a collision fails construction.

WorldConfig enables symbol and unit validation. Symbol validation checks every identifier read by a law. Unit validation compares the inferred expression dimension with the target state unit. Configure units before constructing the world if dimensional validity matters to the run.

DiscreteWorld has the same invariants and evaluates simultaneous updates through the discrete simulator. It does not represent a mixed discrete/continuous model.