The explanation used to be a section inside the Key sheet. It is a standalone
HowBestValueSheet now, opened from a tappable row with a chevron: at the bottom of
the Key sheet, and at the bottom of the More info sheet below the Best Value card.
Gating: the More info row sits INSIDE the same `if let fuelSavings` as the card it
follows, so it disappears with the feature. Verified with Best Value toggled off in
Settings: the More info sheet then shows neither the card nor the row. The Key
sheet row is deliberately ungated — that sheet is the legend and is reachable
whatever the feature is doing.
Copy is unchanged: the same Localizable.strings strings are reused, and the old
section header became the sheet's title. A DEBUG-only `-showBestValueInfo` hook
opens the sheet without a tap.
