Use four-decimal fractional quantity steps

[Pigbibi]

Summary

• set the opt-in IBKR fractional quantity step to 0.0001 shares
• update fractional execution expectations and README examples
• update QPK and strategy package pins to the fractional-step releases

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

Note

IBKR fractional sizing remains opt-in because TWS/IB Gateway API support can depend on account/API path. Whole-share sizing stays the default.