Register market regime control plugin

[Pigbibi]

Summary

• add platform contract for market_regime_control
• add alert guidance and supported strategy validation
• export the new plugin constants from common helpers

Validation

• slowrun bash -lc ".venv/bin/python -m pytest tests/test_strategy_plugins.py -q"
• .venv/bin/python -m ruff check touched platform files

[Pigbibi]

Added version-aware plugin registry support.

Highlights:

• StrategyPluginDefinition now carries supported_schema_versions, default_schema_version, deprecated, and successor_plugin.
• mounts can optionally set expected_schema_version; artifact loading validates it.
• market_regime_control.v1 is the default contract and supports TQQQ + SOXL; legacy plugins remain accepted but deprecated.

Validation:

• slowrun bash -lc ".venv/bin/python -m pytest tests/test_strategy_plugins.py -q"
• slowrun bash -lc ".venv/bin/ruff check src/quant_platform_kit/common/strategy_plugins.py src/quant_platform_kit/common/__init__.py tests/test_strategy_plugins.py"