test: pin the rotation contract and the missing-field default

all eight wl_output variants against their degrees, and a snapshot
serialized without the field deserializing to 0, so an older probe
child never forces a lockstep upgrade.

src/platform/linux.rs

@@ -545,6 +545,38 @@ pub fn get_home_dir_trusted() -> Option<PathBuf> {
 mod tests {
     use super::*;
 
+    #[test]
+    fn test_transform_degrees_maps_all_eight_variants() {
+        use sctk::reexports::client::protocol::wl_output::Transform;
+        // Flipped variants report their rotation: the frame still needs that turn to read
+        // upright, and the mirror half has no producer among desktop compositors to test.
+        for (t, deg) in [
+            (Transform::Normal, 0),
+            (Transform::_90, 90),
+            (Transform::_180, 180),
+            (Transform::_270, 270),
+            (Transform::Flipped, 0),
+            (Transform::Flipped90, 90),
+            (Transform::Flipped180, 180),
+            (Transform::Flipped270, 270),
+        ] {
+            assert_eq!(transform_degrees(t), deg, "{t:?}");
+        }
+    }
+
+    #[test]
+    fn test_display_info_without_transform_defaults_to_zero() {
+        // A snapshot serialized by an older probe child carries no transform field; it must
+        // deserialize with 0 rather than fail, or a greeter-side child update becomes a
+        // lockstep upgrade.
+        let old = r#"{"name":"HDMI-1","x":0,"y":0,"width":1920,"height":1080,"logical_size":null,"refresh_rate":60}"#;
+        let info: WaylandDisplayInfo = serde_json::from_str(old).unwrap();
+        assert_eq!(info.transform, 0);
+        let roundtrip: WaylandDisplayInfo =
+            serde_json::from_str(&serde_json::to_string(&info).unwrap()).unwrap();
+        assert_eq!(roundtrip.transform, 0);
+    }
+
     #[test]
     fn test_run_cmds_trim_newline() {
         assert_eq!(run_cmds_trim_newline("echo -n 123").unwrap(), "123");