fix(derive): keep missing-generation diagnostics author-facing

Replace OUT_DIR/path-shaped proc-macro errors with generated slhx file/module language while preserving the build.rs fix hint.

req: dx/005

req: component/004

slhx-derive/tests/compile_fail.rs

@@ -218,6 +218,10 @@ fn create() -> impl slhx::IntoEffect {
         stderr.contains("add slhx_build::app().run()? to build.rs"),
         "missing build.rs hint in stderr:\n{stderr}"
     );
+    assert!(
+        !stderr.contains("OUT_DIR"),
+        "diagnostic should not teach Cargo internals:\n{stderr}"
+    );
 }
 
 #[test]
@@ -810,6 +814,10 @@ pub mod ui {}
         stderr.contains("add slhx_build::app().run()? to build.rs"),
         "missing build.rs hint in stderr:\n{stderr}"
     );
+    assert!(
+        !stderr.contains("OUT_DIR"),
+        "diagnostic should not teach Cargo internals:\n{stderr}"
+    );
 }
 
 #[test]