feat: Add Bollinger Bands factor

Implements BollingerBandsFactor to compute z-score of price vs rolling mean and std.
Includes tests and integration into factors module.
Closes #3

Author: Satvik-Singh192

src/quant_research_starter/factors/__init__.py

@@ -5,6 +5,8 @@
 from .size import SizeFactor
 from .value import ValueFactor
 from .volatility import IdiosyncraticVolatility, VolatilityFactor
+from .bollinger import BollingerBandsFactor
+
 
 __all__ = [
     "Factor",