Feature: Factory Presets and Traits

[Goldziher]

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.