* A `WP_Site` object initially matches a row from `wp_blogs`.
* A site can be retrieved by its ID through `WP_Site::get_instance()`.
* Adds `sites` to the global cache group and captures instance lookups.
* The multisite bootstrap now ensures `$current_blog` is an instance of `WP_Site`.
Props johnjamesjacoby, jeremyfelt.
See #32450.
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