[codex] Add IBKR paper Top50 switch rehearsal

[Pigbibi]

Summary

• add a paper-only liquidation helper gated by IBKR_PAPER_LIQUIDATE_ONLY and ib_gateway_mode=paper
• add Chinese/English notification keys for paper liquidation status
• document the safe IBKR paper Top50 switch sequence and make CI resolve external stacked-PR dependencies from the base branch

Validation

• git diff --check
• PYTHONPATH=.:../UsEquityStrategies/src:../QuantPlatformKit/src .venv/bin/python -m pytest tests/test_paper_liquidation_service.py tests/test_notifications.py tests/test_runtime_config_support.py -q
• PYTHONPATH=.:../UsEquityStrategies/src:../QuantPlatformKit/src .venv/bin/python -m pytest -q

Notes

• Draft PR stacked on codex/mega-cap-top50-balanced; do not merge before the Top50 platform support PR.
• This only prepares the rehearsal path. It does not execute paper liquidation by itself.