PRODUCT.md described recurring costs as repeating "monthly, every N days,
or every N weeks", understating everyNMonths, which supports an interval
like the other two kinds.
demo-data.ts gave "Rent" both an ordinary monthly purchase and a
never-confirmed recurring cost from 2026-06 onward, so the demo's Rent
Projected read as double-charged for June through September. The ordinary
purchase now stops once the recurring cost exists.