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/kanban/src/lib.rs

@@ -64,7 +64,7 @@ mod tests {
             ui::board::slots::notice.text("queued")
         }
 
-        let effect = inspect(create_card(ui::board::forms::create_card.typed::<CreateCard>()));
+        let effect = inspect(create_card(CreateCard::FORM));
 
         assert!(effect.has_slot(ui::board::slots::notice));
         assert!(matches!(effect.ops(), [Effect::Put { payload: Payload::Text(_), .. }]));