Feature: Factory Presets and Traits

## Focused Scope

Add a small presets/traits API for named partial overrides and ordered composition.

## Proposed v1

- Register named presets on a factory.
- Apply one or more presets during build/batch.
- Later presets override earlier presets.
- Explicit build overrides win over presets.
- Keep the implementation sync-first; async behavior should only be added if it naturally fits `buildAsync()`.

## Out of Scope

- State machines or lifecycle transitions.
- Discriminated-union specialization.
- ORM-style association presets.
- A broad variants framework.

Existing `extend()`, `compose()`, kwargs, and hooks already cover many variant use cases; this issue should focus only on the missing named-preset ergonomics.