Favourites are fuel-scoped: starring pins the station for one fuel only

Fixing the bug where favouriting an Unleaded entry also created a Diesel
favourite. Favourites are now [FavouriteEntry] = (station, fuel) pairs:
- Starring a row pins it only for the fuel being viewed
- Favourites tabs show only fuels that actually have favourites
- Geofence monitor gives priority slots only to favourites matching the
  monitored fuel
- Tests updated + new fuel-scoping test (31/31 passing)

Shared/FuelStore.swift

@@ -17,6 +17,16 @@ struct Coordinate: Equatable, Codable {
     let lng: Double
 }
 
+/// A favourite = a station pinned for ONE fuel type. Starring a row while
+/// viewing Unleaded only creates an Unleaded favourite, so the same station
+/// can be favourite for Diesel independently (or not at all).
+struct FavouriteEntry: Identifiable, Codable, Equatable {
+    var station: FuelStation
+    let fuel: FuelType
+
+    var id: String { "\(fuel.rawValue)|\(station.id)" }
+}
+
 enum SortMode: String, Codable, CaseIterable, Identifiable {
     case cheapest
     case closest
@@ -234,7 +244,7 @@ struct FuelStore {
     static let sortModeKey = "fuelboard.sortMode"      // SortMode raw value
     static let stationLimitKey = "fuelboard.stationLimitMiles" // Int miles (5/10/15)
     static let distanceUnitKey = "fuelboard.distanceUnit"      // DistanceUnit raw value
-    static let favouritesKey = "fuelboard.favourites"      // [FuelStation] JSON
+    static let favouritesKey = "fuelboard.favourites"      // [FavouriteEntry] JSON
     static let alertsEnabledKey = "fuelboard.alertsEnabled" // Bool
     static let alertsRadiusKey = "fuelboard.alertsRadius"   // Double km
     static let onboardingCompletedKey = "fuelboard.onboardingCompleted" // Bool
@@ -348,24 +358,27 @@ struct FuelStore {
 
     // MARK: Favourites
 
-    static func loadFavourites() -> [FuelStation] {
+    /// A favourite pins a station FOR ONE fuel type. Starring a row while
+    /// viewing Unleaded only creates an Unleaded favourite — the station is
+    /// not automatically favourited for Premium or Diesel.
+    static func loadFavourites() -> [FavouriteEntry] {
         if let data = keychainData(service: favouritesKey),
-           let favs = try? JSONDecoder().decode([FuelStation].self, from: data) {
-            return favs.map { fav in
-                var f = fav
-                f.name = fav.name.sanitizedStationTitle
-                return f
+           let entries = try? JSONDecoder().decode([FavouriteEntry].self, from: data) {
+            return entries.map { entry in
+                var e = entry
+                e.station.name = entry.station.name.sanitizedStationTitle
+                return e
             }
         }
         if let defaults = UserDefaults(suiteName: appGroupSuite),
            let data = defaults.data(forKey: favouritesKey),
-           let favs = try? JSONDecoder().decode([FuelStation].self, from: data) {
-            return favs
+           let entries = try? JSONDecoder().decode([FavouriteEntry].self, from: data) {
+            return entries
         }
         return []
     }
 
-    static func saveFavourites(_ favourites: [FuelStation]) {
+    static func saveFavourites(_ favourites: [FavouriteEntry]) {
         if let data = try? JSONEncoder().encode(favourites) {
             UserDefaults(suiteName: appGroupSuite)?.set(data, forKey: favouritesKey)
             writeKeychain(data: data, service: favouritesKey)
@@ -374,14 +387,12 @@ struct FuelStore {
 
     /// Returns favourites with fresh prices applied from the given station list
     /// (favourites keep their cached snapshot when not in the current results).
-    static func refreshedFavourites(_ favourites: [FuelStation], from stations: [FuelStation]) -> [FuelStation] {
-        var updated = favourites
-        for (i, fav) in favourites.enumerated() {
-            if let fresh = stations.first(where: { $0.id == fav.id }) {
-                updated[i] = fresh
-            }
+    /// The fuel scoping is preserved per entry.
+    static func refreshedFavourites(_ favourites: [FavouriteEntry], from stations: [FuelStation]) -> [FavouriteEntry] {
+        favourites.map { entry in
+            guard let fresh = stations.first(where: { $0.id == entry.station.id }) else { return entry }
+            return FavouriteEntry(station: fresh, fuel: entry.fuel)
         }
-        return updated
     }
 
     // MARK: Alerts