Fix HexTile centerline cell selection

Evaluate the third cube coordinate symmetrically to retain the intended centerline tie. Keep the existing pixel oracle and tolerance.

Scope the Mem fixture mutex unwrap allowance so the original strict CI can validate this base.

Co-authored-by: Codex <codex@openai.com>

crates/script/src/tests_services.rs

@@ -11,6 +11,8 @@ use crate::run_code;
 #[derive(Default)]
 struct Mem(Mutex<HashMap<String, Vec<u8>>>);
 
+// A poisoned fixture mutex should fail the test.
+#[allow(clippy::unwrap_used)]
 impl Services for Mem {
     fn read_file(&self, path: &str) -> std::io::Result<Vec<u8>> {
         self.0.lock().unwrap().get(path).cloned().ok_or_else(|| std::io::Error::new(std::io::ErrorKind::NotFound, path.to_string()))