Summary

• share quantity-step env parsing through QuantPlatformKit and update the pin
• use position quantity for zero-target sells to avoid residual shares from quote drift
• document IBKR fractional-share runtime env vars and precedence

Tests

• ./.venv/bin/python -m pytest tests/test_runtime_config_support.py tests/test_execution_service.py::test_execute_rebalance_can_submit_fractional_buy_when_quantity_step_allows tests/test_execution_service.py::test_execute_rebalance_zero_target_sell_uses_position_quantity

Holding merge until CI and Codex review comments are addressed.