feat(derive): expose checked form resource constants

Have #[slhx::form] emit an associated FORM resource so checked form handlers can be called with TodoInput::FORM-style values instead of generated_resource.typed::<T>() noise.

req: form/001

req: form/004

req: form/006

req: codegen/004

examples/techdemo/src/lib.rs

@@ -55,9 +55,7 @@ mod tests {
             ui::control_center::slots::notice.text("queued")
         }
 
-        let batch = inspect(launch_work(
-            ui::control_center::forms::launch_work.typed::<LaunchWork>(),
-        ));
+        let batch = inspect(launch_work(LaunchWork::FORM));
 
         assert!(batch.has_slot(ui::control_center::slots::notice));
         assert!(matches!(batch.ops(), [Effect::Put { payload: Payload::Text(_), .. }]));