Weather-normalized usage (heating/cooling degree-days)

[delabrcd]

Problem

Raw month-to-month usage comparisons are misleading because weather varies year to year. With full historical weather (#1) we can normalize.

Proposal

• Compute heating degree-days (HDD) and cooling degree-days (CDD) per bill period from daily temps (base 65°F, configurable).
• Add a "usage per degree-day" metric and a weather-normalized usage view so months/years are comparable and real efficiency changes are visible.
• Keep the math pure + unit-tested (lib/series.ts or lib/weather/).

Acceptance criteria

• HDD/CDD derived per month; a chart shows weather-normalized usage.
• Hand-calculated test for the degree-day rollup.

Blocked by #1 (needs daily historical temps).