Clarify Best Value route confidence

Shared/FuelStore.swift

@@ -1343,6 +1343,29 @@ struct FuelStore {
         roadDistanceEntry(for: station, userLat: userLat, userLng: userLng) != nil
     }
 
+    /// True only for current-format route cache entries with a separately
+    /// measured return route. Legacy one-way entries are still usable for journey
+    /// arithmetic (`outward × 2`) but Best Value must describe them as estimated.
+    static func hasMeasuredReturnRoadDistance(for station: FuelStation, userLat: Double, userLng: Double) -> Bool {
+        roadDistanceEntry(for: station, userLat: userLat, userLng: userLng)?.returnMeters != nil
+    }
+
+    static func roadDistanceDiagnostics(for station: FuelStation, userLat: Double, userLng: Double) -> RoadDistanceDiagnostics {
+        let displayKM = displayDistanceKM(station: station, userLat: userLat, userLng: userLng)
+        let entry = roadDistanceEntry(for: station, userLat: userLat, userLng: userLng)
+        let outward = entry?.meters
+        let returnMeters = entry?.returnMeters
+        return RoadDistanceDiagnostics(
+            displayDistanceKM: displayKM,
+            displaySource: outward == nil ? .straightLine : .roadRoute,
+            outwardMeters: outward,
+            returnMeters: returnMeters,
+            totalJourneyMeters: outward.map { $0 + (returnMeters ?? $0) },
+            hasMeasuredReturn: returnMeters != nil,
+            hasValidCacheEntry: entry != nil
+        )
+    }
+
     private static func roadDistanceEntry(for station: FuelStation, userLat: Double, userLng: Double) -> CachedRoadDistance? {
         guard let cache = loadRoadDistances(),
               let entry = cache.entries[station.id] else { return nil }
@@ -1432,6 +1455,18 @@ struct FuelStore {
     }
 }
 
+/// Lightweight description of the distance value shown for a station and any
+/// usable road-route cache behind it. No user coordinates are included.
+struct RoadDistanceDiagnostics: Equatable {
+    let displayDistanceKM: Double
+    let displaySource: FuelSavingsDistanceBasis
+    let outwardMeters: Double?
+    let returnMeters: Double?
+    let totalJourneyMeters: Double?
+    let hasMeasuredReturn: Bool
+    let hasValidCacheEntry: Bool
+}
+
 /// Cached Apple-Maps road distances for nearby stations (see
 /// A single cached road distance plus the station pin it was routed to. Keeping
 /// the pin lets `roadDistanceMeters` refuse to serve a route computed for a