test(derive): cover surface inline requirement

Add compile-fail coverage for #[slhx::surface] on non-inline modules so the generated surface boundary stays explicit.

req: codegen/001

req: test/003

slhx-derive/tests/compile_fail.rs

@@ -212,6 +212,44 @@ pub mod ui {}
     );
 }
 
+#[test]
+fn surface_macro_requires_inline_module() {
+    // req: codegen/001 req: test/003
+    let fixture = Fixture::new("slhx-derive-surface-inline-fail");
+    fixture.write(
+        "Cargo.toml",
+        &format!(
+            r#"[package]
+name = "slhx-derive-surface-inline-fail"
+version = "0.0.0"
+edition = "2021"
+
+[lib]
+path = "src/lib.rs"
+
+[dependencies]
+slhx = {{ path = {:?} }}
+"#,
+            repo_path("slhx")
+        ),
+    );
+    fixture.write(
+        "src/lib.rs",
+        r#"#[slhx::surface]
+pub mod ui;
+"#,
+    );
+
+    let output = check_fixture(&fixture);
+
+    assert!(!output.status.success(), "fixture unexpectedly compiled");
+    let stderr = String::from_utf8_lossy(&output.stderr);
+    assert!(
+        stderr.contains("#[slhx::surface] must be used on an inline module"),
+        "missing inline-module diagnostic in stderr:\n{stderr}"
+    );
+}
+
 #[test]
 fn generated_resource_references_fail_when_name_is_absent() {
     // req: style/002 req: codegen/001 req: test/003