Qt 6.8 re-notifies a var binding that re-evaluates to the same object,
so a shallow provider copy rebuilt the cost chart points even though the
daily rows were unchanged. Qt 6.11, used in CI, suppresses that
notification, which hid the popup-cost-details smoke failure there.

Memoize the points on the daily rows, number format, and metric through
a caller-owned memo in ProviderCostSection.

Co-authored-by: Claude Opus 5.5 (1M context) <noreply@anthropic.com>