Implemented configurable display format (mean/median/both) while always
calculating and exposing both price_mean and price_median attributes.
Core changes:
- utils/average.py: Refactored calculate_mean_median() to always return both
values, added comprehensive None handling (117 lines changed)
- sensor/attributes/helpers.py: Always include both attributes regardless of
user display preference (41 lines)
- sensor/core.py: Dynamic _unrecorded_attributes based on display setting
(55 lines), extracted helper methods to reduce complexity
- Updated all calculators (rolling_hour, trend, volatility, window_24h) to
use new always-both approach
Impact: Users can switch display format in UI without losing historical data.
Automation authors always have access to both statistical measures.