Stations sort: "Worth the Drive" segment is now "Best Value"

The third sort segment, the caption line it drives on the Stations tab, and the
Settings sentence about sorting now read "Best Value" — that is
SortMode.worthTheDrive.displayName plus the two copy sites that name the sort.

The feature that backs it keeps the name "Worth the Drive": the Settings toggle
("Enable Worth the Drive"), the Fuel Savings section header and footer, the sheet
header "Worth the drive?", the "Worth the Drive unavailable: …" text and
FuelSavingsStatus.worthTheDrive's own wording are all unchanged.

The enum's raw value stays `worthTheDrive`, so stored sort preferences and widget
configs keep decoding — only the label moved.

3 new tests pin the three sort names, the stability of the raw values, and the
fact that the feature's own wording did not change (172 total, 0 failures).

Verified on the simulator: the Stations segmented control reads
Closest | Cheapest | Best Value.

FuelBoard/StationsView.swift

@@ -49,7 +49,7 @@ struct StationsView: View {
         switch sortMode {
         case .closest: mode = "Closest"
         case .cheapest: mode = "Cheapest"
-        case .worthTheDrive: mode = "Worth the Drive"
+        case .worthTheDrive: mode = "Best Value"
         }
         let tapHint = stationTapAction == .openMap
             ? "tap a station for directions."