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

slhx-derive/tests/compile_fail.rs

@@ -423,6 +423,10 @@ struct CreateTodo {
 fn create(_form: slhx::Form<CreateTodo>) -> impl slhx::IntoEffect {
     slhx::event("created", "")
 }
+
+fn smoke() {
+    let _ = create(CreateTodo::FORM);
+}
 "#,
     );