Feature: Lazy Evaluation Ergonomics

## Focused Scope

Improve lazy-evaluation ergonomics and documentation around the existing `use()` API.

## Proposed v1

- Document `use(async () => ...)` with `buildAsync()`.
- Add tests showing async `use()` resolves through `buildAsync()`.
- Ensure sync `build()` fails clearly when async lazy values are used.
- Optionally add `lazy` as an alias for `use` if the naming improvement is worth the public API surface.

## Out of Scope

- Field dependency graphs.
- `lazyIf()` or conditional DSLs.
- New relationship/association semantics.

Conditional values can already be expressed in the factory function through kwargs and hooks.