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/build.rs

@@ -0,0 +1,4 @@
+fn main() {
+    let out = std::path::PathBuf::from(std::env::var_os("OUT_DIR").unwrap());
+    std::fs::write(out.join("slhx.generated.rs"), "").unwrap();
+}