fix: use self_alloc provenance for parent pointers in cursor

Author: avrabe

src/cursor.rs

@@ -256,11 +256,19 @@ impl NodeData {
         green: Green,
         mutable: bool,
     ) -> ptr::NonNull<NodeData> {
+        // Extract parent's self_alloc before wrapping in ManuallyDrop.
+        // We read it while we still OWN the SyntaxNode (before any shared refs).
+        let parent_alloc = parent.as_ref().map(|p| {
+            // p is &SyntaxNode from owned Option — safe to read here.
+            // UnsafeCell::get gives *mut without freezing.
+            let node_data = unsafe { *p.ptr.get() };
+            unsafe { ptr::NonNull::new_unchecked((*node_data.as_ptr()).self_alloc) }
+        });
         let parent = ManuallyDrop::new(parent);
         let res = NodeData {
             _c: Count::new(),
             rc: Cell::new(1),
-            parent: Cell::new(parent.as_ref().map(|it| it.ptr())),
+            parent: Cell::new(parent_alloc),
             index: Cell::new(index),
             green,