bd-source-adapter

Add a new source adapter to better-data — code that reads from a WordPress data store the library doesn't cover yet (comments, attachments, transients, custom tables). Mirror the canonical shape PostSource / UserSource / TermSource use — a non-final class with static hydrate(int|object, $dtoClass) and hydrateMany(int[], $dtoClass) methods. Critical contract — the meta fetcher closure passed to the AttributeDrivenHydrator must return null when the meta key does not exist (use metadata_exists guard) and the stored value otherwise. Without this guard you cannot distinguish "missing meta → use default" from "stored empty string → preserve emptiness", and Reflection-default fallback breaks. Bulk hydration must prewarm caches with the equivalent of update_meta_cache + (where applicable) _prime_post_caches. Use when integrating a new WP store. Triggers on creating a class in src/Source/, hydrate / hydrateMany method signatures, references to AttributeDrivenHydrator from a source.