chore: import upstream snapshot with attribution

tests/fixtures/sample_callback_refs.py

@@ -0,0 +1,35 @@
+"""Fixture for issue #363: function references in callback positions.
+
+Each `*_callback` function is passed as a bare-identifier argument to
+another call. They are never invoked with parens, so without REFERENCES
+edge tracking they would be flagged as dead code.
+"""
+from concurrent.futures import ThreadPoolExecutor
+
+
+def executor_callback():
+    return "submitted"
+
+
+def filter_callback(item):
+    return item > 0
+
+
+def map_callback(item):
+    return item * 2
+
+
+def trigger_executor():
+    with ThreadPoolExecutor() as executor:
+        future = executor.submit(executor_callback)
+        return future
+
+
+def trigger_filter():
+    items = [1, -2, 3, -4]
+    return list(filter(filter_callback, items))
+
+
+def trigger_map():
+    items = [1, 2, 3]
+    return list(map(map_callback, items))