Guard zero-equity value target translation

[Pigbibi]

Summary

• mark value-target decisions as no_execute when runtime portfolio_total_equity is non-positive
• avoid translating value targets to weights with total_equity=0 while preserving diagnostics
• add regression coverage for empty/new accounts

Validation

• PYTHONPATH=.:../QuantPlatformKit/src:../UsEquityStrategies/src:../HkEquityStrategies/src uv run --no-project --with pytest --with pandas pytest tests/test_decision_mapper.py -q
• PYTHONPATH=.:../QuantPlatformKit/src:../UsEquityStrategies/src:../HkEquityStrategies/src uv run --no-project --with ruff ruff check decision_mapper.py tests/test_decision_mapper.py