What is "variance"?

I see this attribute applies to many different AST node types:
```ts
variance: K.VarianceKind | "plus" | "minus" | null;
```
What does it do? What does it look like in the parsed/emitted JavaScript source code?

No file or test is named. Start by tracing the variance attribute across the AST node types, then compare representative parsed and emitted JavaScript to determine its effect. Document what the attribute means, where it applies, and what output users should expect.

Written by the indexing model from the issue text.