Use four-decimal fractional quantity steps

[Pigbibi]

Summary

• set the LongBridge default fractional quantity step to 0.0001 shares
• update SOXX fractional-buy expectations to four-decimal sizing
• update QPK and strategy package pins to the fractional-step releases
• refresh README examples for the platform-supported quantity step

Tests

• ./.venv/bin/python -m pytest tests/test_runtime_config_support.py tests/test_runtime_composer.py tests/test_rebalance_service.py::RebalanceServiceNotificationTests::test_fractional_quantity_step_allows_small_soxx_target_buy tests/test_rebalance_service.py::RebalanceServiceNotificationTests::test_zero_target_sell_uses_sellable_quantity_not_price_derived_floor tests/test_rebalance_service.py::RebalanceServiceNotificationTests::test_fractional_buy_uses_budget_when_broker_estimate_is_whole_share_zero