Treat `T[keyof T]` as covariant (rather than invariant) during variance measurement

Suggestion

See title. Will fill in more details later.

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✅ Viability Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

A lot of mapped types are covariant by construction, but we're not always able to determine that

📃 Motivating Example

💻 Use Cases

See incoming linked issues

The issue proposes treating T[keyof T] as covariant during variance measurement and notes that mapped types are not always recognized as covariant. No source files, tests, or compiler entry points are identified, so begin by locating variance measurement and mapped-type handling in the TypeScript compiler. Done means the intended covariance behavior is specified and covered by appropriate type-checking tests.