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/src/lib.rs

@@ -66,13 +66,22 @@ fn inject_surface_include(item: TokenStream) -> TokenStream {
 
 fn surface_include() -> String {
     let Some(path) = generated_path("slhx.generated.rs") else {
-        return String::new();
+        return format!(
+            " compile_error!({:?}); ",
+            "#[slhx::surface] requires OUT_DIR; run inside a Cargo crate with slhx_build::app() in build.rs"
+        );
     };
 
     if path.exists() {
         format!(" include!({:?}); ", path.display().to_string())
     } else {
-        String::new()
+        format!(
+            " compile_error!({:?}); ",
+            format!(
+                "#[slhx::surface] could not find {}; add slhx_build::app().run()? to build.rs or check template generation",
+                path.display()
+            )
+        )
     }
 }