refactor: switch longbridge to unified strategy runtime

[Pigbibi]

Summary

• switch LongBridge mainline to unified strategy runtime and decision mapper
• remove the platform-local allocation shim and hard-coded allocation path
• add runtime, mapper, request-handling, and rebalance regression coverage

Test Plan

• python3 -m py_compile strategy_loader.py strategy_runtime.py tests/test_strategy_loader.py tests/test_strategy_runtime.py
• python3 -m ruff check strategy_loader.py strategy_runtime.py tests/test_strategy_loader.py tests/test_strategy_runtime.py
• source /tmp/ues-strategy-contract-venv13/bin/activate && PYTHONPATH=.:/Users/lisiyi/Projects/QuantPlatformKit/src:/Users/lisiyi/Projects/UsEquityStrategies/src python -m unittest tests.test_strategy_loader tests.test_strategy_runtime tests.test_rebalance_service tests.test_request_handling -v