Runtime: force each step to return a state object

The runtime right now is generous about return types from expressions. You can return anything: objects, arrays, primitives, falsy values.

I think we need, in the worker at least, to enforce that a step returns an OBJECT type.

Here's the thing. A dataclip MUST be an object. That's what a dataclip is.

Technically in the runtime, a step can return anything. This is correct and fine.

I just think we need a mode where the runtime enforces the return of an object from each step, or throws an exception. The worker would enable this mode.

This lets us remove an awkward fix in lightning where scalar return values are wrapped in a { value } object, which is really messy.

There was a debate around this but I'm struggling to find the issue. Might be in lightning somewhere.

Start by tracing the runtime step return handling and how the worker invokes it, then locate the Lightning code that wraps scalar returns in a { value } object. Define how an object-only mode should be enabled by the worker, how invalid returns should fail, and how existing runtime behavior remains unchanged; verify the behavior with the relevant runtime and worker tests, if present.