domain: add Sidecar Sync entities and ports (SidecarRecord, SidecarConfig, SidecarWriterPort)

crates/domain/tests/entities/mod.rs

@@ -15,3 +15,4 @@ mod album;
 mod tag;
 mod share_scope;
 mod share_link;
+mod sidecar_record;

crates/domain/tests/entities/sidecar_record.rs

@@ -0,0 +1,31 @@
+use domain::entities::{SidecarRecord, SyncStatus};
+use domain::value_objects::{Checksum, SystemId};
+
+#[test]
+fn new_is_pending_write() {
+    let record = SidecarRecord::new(SystemId::new(), "/path/to/sidecar.xmp");
+    assert_eq!(record.sync_status, SyncStatus::PendingWrite);
+    assert!(record.last_synced_at.is_none());
+    assert!(record.last_known_file_hash.is_none());
+}
+
+#[test]
+fn sync_lifecycle() {
+    let mut record = SidecarRecord::new(SystemId::new(), "/path/to/sidecar.xmp");
+
+    let hash = Checksum::new("a".repeat(64)).unwrap();
+    record.mark_synced(hash);
+    assert_eq!(record.sync_status, SyncStatus::InSync);
+    assert!(record.last_synced_at.is_some());
+    assert!(record.last_known_file_hash.is_some());
+
+    record.mark_pending_read();
+    assert_eq!(record.sync_status, SyncStatus::PendingRead);
+
+    record.mark_conflict();
+    assert_eq!(record.sync_status, SyncStatus::Conflict);
+
+    record.mark_error("disk full");
+    assert_eq!(record.sync_status, SyncStatus::Error);
+    assert_eq!(record.error_message.as_deref(), Some("disk full"));
+}