ractor.c: preserve aliasing during Ractor move

Moving an unshareable object graph with duplicate references failed to
preserve aliasing because the traversal checked the `rec` table too late.

On a move path, `move_leave` replaces the original object with a
`Ractor::MovedObject` tombstone. Because `move_enter` classifies this
tombstone as shareable, the second visit returned `traverse_skip` before
the visited `rec` lookup was reached.

Fix this by performing the `st_lookup` on `data->rec` at the top of
`obj_traverse_replace_i`, before calling `enter_func`. We guard the lookup
with a NULL check on `data->rec` to avoid allocating `rec_hash` for
simple graphs.

Author: ianks

bootstraptest/test_ractor.rb

@@ -656,6 +656,29 @@ def check obj1
   end
 }
 
+# move preserves aliasing inside the moved object graph
+assert_equal 'true', %q{
+  r = Ractor.new do
+    Ractor.receive
+  end
+
+  leaf = +"leaf"
+  moved = r.send([leaf, leaf], move: true).value
+  moved[0].equal?(moved[1])
+}
+
+# move handles cyclic references safely and preserves aliasing
+assert_equal 'true', %q{
+  r = Ractor.new do
+    Ractor.receive
+  end
+
+  a = []
+  a << a
+  moved = r.send(a, move: true).value
+  moved.equal?(moved[0])
+}
+
 # unshareable frozen objects should still be frozen in new ractor after move
 assert_equal 'true', %q{
   r = Ractor.new do

ractor.c

@@ -1773,23 +1773,21 @@ obj_traverse_replace_i(VALUE obj, struct obj_traverse_replace_data *data)
         return 0;
     }
 
+    if (UNLIKELY(data->rec && st_lookup(data->rec, (st_data_t)obj, &replacement))) {
+        data->replacement = (VALUE)replacement;
+        return 0;
+    }
+
     switch (data->enter_func(obj, data)) {
       case traverse_cont: break;
       case traverse_skip: return 0; // skip children
       case traverse_stop: return 1; // stop search
     }
 
     replacement = (st_data_t)data->replacement;
-
-    if (UNLIKELY(st_lookup(obj_traverse_replace_rec(data), (st_data_t)obj, &replacement))) {
-        data->replacement = (VALUE)replacement;
-        return 0;
-    }
-    else {
-        st_insert(obj_traverse_replace_rec(data), (st_data_t)obj, replacement);
-        RB_OBJ_WRITTEN(data->rec_hash, Qundef, obj);
-        RB_OBJ_WRITTEN(data->rec_hash, Qundef, replacement);
-    }
+    st_insert(obj_traverse_replace_rec(data), (st_data_t)obj, replacement);
+    RB_OBJ_WRITTEN(data->rec_hash, Qundef, obj);
+    RB_OBJ_WRITTEN(data->rec_hash, Qundef, replacement);
 
     if (!data->move) {
         obj = replacement;