fix(derive): fail missing surface generation

Make #[slhx::surface] report a compile-time error when slhx.generated.rs is absent instead of silently expanding to an empty module. Add compile-fail coverage for the missing generated include path.

req: build/004

req: test/003

slhx-derive/tests/surface.rs

@@ -6,6 +6,6 @@ mod ui {
 }
 
 #[test]
-fn surface_macro_preserves_inline_module_without_generated_file() {
+fn surface_macro_preserves_inline_module_with_generated_file() {
     assert_eq!(ui::EXISTING, 1);
 }