Fix runner.py test failures (test_runner.py: 5 failed, 5 errors)

Issues:\n1. SimulationConfig dataclass lacks tickers/start_date/end_date kwargs used by tests\n2. concentration_guard thresholds too strict for equal-weight portfolios (33% each > 5% limit)\n3. PaperTradingSession constructor doesn’t initialize positions from config tickers\n4. run_day/test fixtures crash on empty state