Add Worth the Drive fuel savings with road-routed estimates

Optional Fuel Savings feature (off by default): vehicle MPG in UK imperial gallons, fill amount in pounds or litres, persisted through the existing keychain/app-group store. Ranks the WHOLE station pool by estimated net saving (fuel-price saving at each pump minus the extra return-trip driving cost) using MapKit road distances with a straight-line fallback marked (est.).

The closest station with a valid price for the selected fuel is the comparison baseline and is labelled "Closest station"; other rows read "Save £X.XX after travel" / "£X.XX worse off after travel" / "Break even". Labels live in the shared model so they are unit-tested. The TOP badge still marks the cheapest pump price and reads "CHEAPEST" when the list is ranked by net saving.

Road routing now prioritises the Worth-the-Drive candidates plus the baseline and merges cache entries instead of replacing them, so far-out stations get real routes rather than permanent straight-line estimates. Numeric fields gain a Done button above the keypad (decimalPad has no Return key); the fill-up control is hidden unless the feature is configured; Favourites shares one session fill amount with the station list. 131 tests.

FuelBoardWidgets/FuelPriceWidget.swift

@@ -242,8 +242,9 @@ struct FuelPriceTimelineProvider<Configuration: WidgetConfigurationIntent & Widg
                     }
                     return lhs.prices[fuel]! < rhs.prices[fuel]!
                 }
-            case .cheapest:
-                // Cheapest first; distance only breaks ties.
+            case .cheapest, .worthTheDrive:
+                // Cheapest first; distance only breaks ties. Worth-the-Drive is app-only;
+                // widgets fall back to cheapest ordering if a stored config ever carries it.
                 ordered = filtered.sorted { lhs, rhs in
                     let lPrice = lhs.prices[fuel]!
                     let rPrice = rhs.prices[fuel]!