refactor: finalize shared strategy contract cleanup

[Pigbibi]

Summary

• add shared strategy contracts, validators, loader support, and migration notes
• remove deprecated QuantPlatformKit compatibility helpers after the compatibility window
• update contract and regression tests for the unified catalog/policy path

Test Plan

• python3 -m py_compile src/quant_platform_kit/common/strategy_contracts.py src/quant_platform_kit/strategy_contracts.py src/quant_platform_kit/init.py tests/test_strategy_contracts.py
• python3 -m ruff check src/quant_platform_kit/common/strategy_contracts.py src/quant_platform_kit/strategy_contracts.py src/quant_platform_kit/init.py tests/test_strategy_contracts.py
• PYTHONPATH=src python3 -m unittest tests.test_strategy_contracts -v