Add per-network super-admin scoping to REST sites controller - #8

Summary

• Adds per-network super-admin scoping (is_super_admin( $user_id, $network_id ), get_super_admins( $network_id )) so super-admin status can be checked against a specific network, not just the current one.
• Uses this in the wp/v2/sites REST controller's permission checks (check_network_ids_exist() / check_network_access()) for the list, single-item, create, and update routes, so a network's own site_admins option can scope access to sites on that network.
• Adds PHPUnit coverage for the new network-id validation and cross-network access behavior on those routes.

Test plan

• npm run test:php -- --filter WP_Test_REST_Sites_Controller (single-site)
• npm run test:php -- -c tests/phpunit/multisite.xml --filter WP_Test_REST_Sites_Controller (multisite)
• Manual REST testing against a multi-network install for permission checks (super admin vs. non-super-admin, cross-network access)

🤖 Generated with Claude Code