* Support frozen ElementTree attribute mappings
Preserve assigned mapping identity while copying constructor attributes, and use checked union ownership through Element operations.
Assisted-by: OpenAI Codex:gpt-6.1-sol
* Sync final ElementTree runtime and companion tests
Import ElementTree.py from CPython d220409a13b77de7a6c30abe16ea90db01528edc through update_lib migrate. Copy six complete canonical tests with update_lib copy-test, preserving all surrounding test markers. This aligns pure-Python and native frozendict diagnostic expectations and covers the runtime import companion changes.
Assisted-by: OpenAI Codex:model version unavailable
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