fix(engine): gate the temp-name symlink test to unix

`save_fails_upfront_when_the_temp_name_is_taken` calls
`std::os::unix::fs::symlink` but was only gated on non-wasm, so the
engine crate's lib tests did not compile on Windows. Gate it `cfg(unix)`
like the neighbouring permissions test.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/io/tests.rs

@@ -19,6 +19,7 @@ fn copied(wb: &Workbook, range: &str) -> String {
 }
 
 #[cfg(not(target_arch = "wasm32"))]
+#[cfg(unix)]
 #[test]
 fn save_fails_upfront_when_the_temp_name_is_taken() {
     // Another local user who can write the directory must not be able to point the