Old gains now fade in the HV Score

The long-term part of the performance score now uses a time-decayed annualized return, so very old results count less.

• Each day's return is weighted by its age with a 180-day half-life: 6 months ago counts 50%, a year ago 25%, two years ago about 6%.
• The full history is still used. Nothing is cut off; older performance just fades gradually.
• Window weights are unchanged: 30D 20%, 90D 35%, 180D 30%, long-term 15%.
• All vault scores were recalculated.