Best Value: baseline on the closest eligible price and gate on freshness

- Add FuelPriceFreshness: selected-fuel-aware eligibility plus a shared
  comparison confidence, so a saving is presented as verified only when
  both the candidate and the baseline pass the gate for the fuel on screen.
- Rank against the closest station with an eligible selected-fuel price
  instead of the physically closest station, so a single stale neighbour
  can no longer invalidate every comparison in the list. Ordinary Closest
  sorting still includes every priced station.
- Keep ineligible prices visible, fall back to pump-price order, and label
  Best Value unavailable when no comparison qualifies.
- Widget: same baseline and confidence path as the app.
- Tests: stale-closest baseline, alert baseline, eligibility policy,
  boundary and missing-timestamp regressions.

FuelBoardWidgets/FuelPriceWidget.swift

@@ -203,6 +203,8 @@ struct FuelPriceTimelineProvider<Configuration: WidgetConfigurationIntent & Widg
         // 3) Load + order stations per widget mode.
         let ordered: [FuelStation]
         var bestValueResult: FuelSavingsResult?
+        var bestValuePriceConfidence: FuelPriceFreshness.ComparisonConfidence = .ineligible
+        var bestValuePriceWarning: String?
         if isFavourites {
             let favourites = FuelStore.loadFavourites()
                 .filter { $0.station.prices[$0.fuel] != nil }
@@ -306,7 +308,8 @@ struct FuelPriceTimelineProvider<Configuration: WidgetConfigurationIntent & Widg
                     break
                 }
                 let amount = settings.preferredAmount
-                let baseline = FuelSavingsEstimator.baselineStation(filtered, fuel: fuel, journeyMiles: journey)
+                let baseline = FuelSavingsEstimator.bestValueBaselineStation(filtered, fuel: fuel,
+                                                                             journeyMiles: journey)
                 let ranked = FuelSavingsEstimator.ranked(
                     filtered,
                     fuel: fuel,
@@ -317,6 +320,13 @@ struct FuelPriceTimelineProvider<Configuration: WidgetConfigurationIntent & Widg
                 )
                 ordered = ranked
                 if let first = ranked.first, let baseline {
+                    bestValuePriceConfidence = FuelPriceFreshness.comparisonConfidence(
+                        station: first, fuel: fuel, baseline: baseline)
+                    bestValuePriceWarning = FuelPriceFreshness.comparisonWarning(
+                        station: first, fuel: fuel, baseline: baseline)
+                }
+                if let first = ranked.first, let baseline,
+                   bestValuePriceConfidence != .ineligible {
                     let baselineJourney = journey(baseline)
                     let result = FuelSavingsEstimator.availability(
                         station: first,
@@ -348,7 +358,9 @@ struct FuelPriceTimelineProvider<Configuration: WidgetConfigurationIntent & Widg
                 date: Date(), stations: Array(ordered.prefix(8)),
                 fuel: fuel, sort: sort, isFavourites: isFavourites,
                 location: origin, locationSource: source,
-                unit: unit, bestValueResult: bestValueResult
+                unit: unit, bestValueResult: bestValueResult,
+                bestValuePriceConfidence: bestValuePriceConfidence,
+                bestValuePriceWarning: bestValuePriceWarning
             )
         ), originSource)
     }