Replace hardcoded validator class lists with a declarative #[Mixin]
attribute and extract the mixin generation logic into a reusable
CodeGen namespace under src-dev/CodeGen/.
The new MixinGenerator discovers prefix definitions and filtering
rules by scanning #[Mixin] attributes on the target namespace's
classes, removing the need for hardcoded configuration. It supports
configurable interface types (Builder for __callStatic, Chain for
__call) with custom suffixes, return types, and root extends.
This is the first step toward extracting the code generation into a
standalone package that can map __call/__callStatic to any namespace,
possibly for Respect/StringFormatter and any kind of project in the
future.
