ZAPHandler Interface

File	Change
`lib/ZAPHandler.interface.php`	Created — defines `ZAPHandler` interface with `public function getContent()`
`lib/ZAPHome.class.php`	Added `implements ZAPHandler`
`lib/ZAPDisplay.class.php`	Added `implements ZAPHandler`
`lib/ZAPZap.class.php`	Added `implements ZAPHandler`
`lib/ZAPController.class.php`	Added `instanceof ZAPHandler` guard inside `fetch()` before calling `getContent()`

Any class named ZAP{Mode} in lib/ is autoloaded and instantiated by ZAPController
The controller now checks $this->action instanceof ZAPHandler before calling getContent()
If a handler forgets to implement the interface, you get a clear die() message instead of a cryptic runtime error
The interface file uses the .interface.php naming convention, which the existing autoloader already handles (it checks .interface.php first, then .class.php)