Feature: Type Ergonomics Improvements

## Focused Scope

Improve concrete TypeScript ergonomics where the current API can be made more precise.

## Proposed v1

- Export `FactoryOutput`.
- Consider `InferFactorySchema` only if it can be implemented without broad compiler-style promises.
- Improve `extend()` typing so extension functions can return only added fields when possible.
- Add `expect-type` coverage for literal preservation, explicit discriminated unions, and composition not degrading to `any`.

## Out of Scope

- Runtime validation from TypeScript types.
- Custom compiler error messages.
- IntelliSense guarantees.
- Object-literal constructor examples that do not match the current API.

The original issue mixed useful DX work with goals TypeScript cannot provide at runtime. This version keeps the work measurable.