Clean up fractional sizing support

[Pigbibi]

Summary

• share quantity-step env parsing through QuantPlatformKit and update the pin
• sell full sellable quantity when a target is zero, avoiding residual shares from quote drift
• document LongBridge quantity-step precedence and liquidation sizing behavior

Tests

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

Holding merge until CI and Codex review comments are addressed.