Feature: Type Ergonomics Improvements #67

Focused Scope

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

Proposed v1

• Export FactoryOutput<F>.
• Consider InferFactorySchema<S> 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.