Adding a selector to lookbook

A selector picks a subset of size up-to-K from N candidates given the manifest. It implements the Selector protocol from lookbook/base.py:

class Selector(Protocol):
    def select(
        self,
        candidates: Iterable[ImageRef],
        manifest: Manifest,
        k: int,
        constraints: Mapping[str, Any] = (),
    ) -> list[ImageRef]: ...

Read lookbook-dev first if you haven't.

When to write a new selector vs reuse an existing one