feat: enforce JSON-compatible step outputs at construction time

[jumski]

This pull request enforces JSON-compatible step outputs at .step() construction time by adding TypeScript constraints to the step handler functions.

The changes introduce a Json type constraint on step handler return values, preventing non-serializable types like undefined, Symbol, or functions from being returned. A new WidenJson<T> utility type is added to properly widen literal types (e.g., 123 becomes number) while maintaining JSON compatibility.

All six .step() method overloads are updated to use a constrained THandler type parameter instead of a generic TOutput, ensuring type safety at compile time. Comprehensive type tests verify that valid JSON types are accepted while invalid types like undefined and functions are properly rejected with TypeScript errors.