PaperTradingEngine now accepts an optional SnapshotStore and uses it
for run_next_open candidate fetching, ensuring live candidate selection
matches the backtester's pre-computed scores exactly. run_reaction_close
keeps EventDetector for real-time intraday event detection. Adds
load_snapshot_store_for_session() helper with auto-refresh logic.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>