test(derive): cover missing generated resource

Add compile-fail coverage for generated APIs: a surface include that exposes only one class must reject references to absent generated class names during cargo check.

req: codegen/001

req: style/002

req: test/003

slhx-derive/tests/compile_fail.rs

@@ -67,6 +67,66 @@ fn known() {}
     );
 }
 
+#[test]
+fn generated_resource_references_fail_when_name_is_absent() {
+    // req: style/002 req: codegen/001 req: test/003
+    let fixture = Fixture::new("slhx-derive-generated-resource-fail");
+    fixture.write(
+        "Cargo.toml",
+        &format!(
+            r#"[package]
+name = "slhx-derive-generated-resource-fail"
+version = "0.0.0"
+edition = "2021"
+
+[lib]
+path = "src/lib.rs"
+
+[dependencies]
+slhx = {{ path = {:?} }}
+"#,
+            repo_path("slhx")
+        ),
+    );
+    fixture.write(
+        "build.rs",
+        r#"fn main() {
+    let out = std::path::PathBuf::from(std::env::var_os("OUT_DIR").unwrap());
+    std::fs::write(
+        out.join("slhx.generated.rs"),
+        "pub mod classes { pub const card: ::slhx::CssClass = ::slhx::CssClass::new(\"card\"); }\n",
+    )
+    .unwrap();
+}
+"#,
+    );
+    fixture.write(
+        "src/lib.rs",
+        r#"#[slhx::surface]
+pub mod ui {}
+
+pub const KNOWN: slhx::CssClass = ui::classes::card;
+pub const MISSING: slhx::CssClass = ui::classes::missing;
+"#,
+    );
+
+    let output = Command::new("cargo")
+        .arg("check")
+        .arg("--quiet")
+        .arg("--manifest-path")
+        .arg(fixture.path.join("Cargo.toml"))
+        .env("CARGO_TARGET_DIR", fixture.path.join("target"))
+        .output()
+        .expect("cargo check fixture runs");
+
+    assert!(!output.status.success(), "fixture unexpectedly compiled");
+    let stderr = String::from_utf8_lossy(&output.stderr);
+    assert!(
+        stderr.contains("cannot find value `missing` in module `ui::classes`"),
+        "missing generated-resource diagnostic in stderr:\n{stderr}"
+    );
+}
+
 fn repo_path(crate_name: &str) -> PathBuf {
     Path::new(env!("CARGO_MANIFEST_DIR"))
         .parent()