08: Registrar

[nikolaystrikhar]

Adds the sub-plugin registry and its Registrar_Interface contract.

Stacked on #7.

$registrar = new Registrar();
$registrar->register( $sub_plugin );
$registrar->all();   // [ 'give-recurring' => Sub_Plugin ]

• Keyed by slug. Re-registering the same slug replaces in place, so a host registering from two code paths gets one entry rather than two loads, and the order stays stable.
• Load order is now documented. all() returns insertion order and Loader::load_all() require_onces in that order, so registering an add-on before the sub-plugin whose class it extends is a fatal at plugins_loaded. The README says so now, and two tests pin the order.