Add cash buffer branch runtime support

[Pigbibi]

Summary

• add runtime support for cash_buffer_branch_default through the existing IBKR strategy loader / registry path
• add snapshot guard + manifest validation, reconciliation output, and same-day practical idempotency checks
• add Cloud Run env sync support plus dry-run / paper / rollback example env patches
• bundle the canonical cash-buffer config and pin us-equity-strategies to QuantStrategyLab/UsEquityStrategies@ced79a2

Why

The current Cloud Run paper service still runs an image that only supports global_etf_rotation. This PR is the platform-side runtime work needed before IBKR paper can switch to cash_buffer_branch_default in dry-run mode.

Depends on

• Add cash buffer branch default UsEquityStrategies#3

Validation

• PYTHONPATH=/tmp/ibkr-publish:/tmp/ues-publish:/tmp/ues-publish/src:/Users/lisiyi/Projects/QuantPlatformKit/src /Users/lisiyi/Projects/InteractiveBrokersPlatform/.venv/bin/python -m pytest /tmp/ibkr-publish/tests/test_strategy_loader.py /tmp/ibkr-publish/tests/test_runtime_config_support.py /tmp/ibkr-publish/tests/test_snapshot_strategy_runtime.py /tmp/ibkr-publish/tests/test_execution_service.py /tmp/ibkr-publish/tests/test_rebalance_service.py /tmp/ibkr-publish/tests/test_feature_snapshot_service.py /tmp/ibkr-publish/tests/test_cloud_run_entrypoint.py
• 39 passed
• bash /tmp/ibkr-publish/tests/test_sync_cloud_run_env_workflow.sh