[codex] refactor longbridge platform strategy policy

[Pigbibi]

What changed

• move LongBridge strategy registry resolution onto the shared QuantPlatformKit platform policy helper
• keep LongBridge-specific choices declarative: enabled profiles, default profile, rollback profile, supported domains
• pin quant-platform-kit and us-equity-strategies to the companion branch commits for CI

Why

• keep platform repos thin and move shared strategy policy behavior out of LongBridge-specific glue

Impact

• no intended runtime behavior change
• this PR depends on the companion QPK and UES PRs

Validation

• python3 -m py_compile strategy_registry.py
• python3 -m ruff check strategy_registry.py
• PYTHONPATH=... pytest tests/test_runtime_config_support.py tests/test_strategy_loader.py