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)

FuelBoard/FavouritesView.swift

@@ -3,20 +3,21 @@ import SwiftUI
 /// Favourites tab — starred stations, split by fuel type. Each fuel type
 /// with at least one favourite appears as its own tab; that tab's favourites
 /// are ranked cheapest-first for that fuel, with the cheapest called out.
+/// Favourites are fuel-scoped entries — starring a station only pins it for
+/// the fuel you were viewing, so an Unleaded favourite never shows as Diesel.
 struct FavouritesView: View {
-    let favourites: [FuelStation]
+    let favourites: [FavouriteEntry]
     /// The app's currently selected fuel — used only as the initial tab.
     let selectedFuel: FuelType
     let location: Coordinate?
     let distanceUnit: DistanceUnit
-    let favouriteIDs: Set<String>
-    var onToggleFavourite: (FuelStation) -> Void = { _ in }
+    var onToggleFavourite: (FuelStation, FuelType) -> Void = { _, _ in }
 
-    /// Fuel types that currently have at least one favourite selling them —
-    /// these are the only tabs shown.
+    /// Fuel types that currently have at least one favourite — these are the
+    /// only tabs shown (a fuel with no favourites gets no tab).
     private var availableFuels: [FuelType] {
         FuelType.allCases.filter { fuel in
-            favourites.contains { $0.prices[fuel] != nil }
+            favourites.contains { $0.fuel == fuel }
         }
     }
 
@@ -28,9 +29,9 @@ struct FavouritesView: View {
         availableFuels.contains(fuel) ? fuel : (availableFuels.first ?? .e10)
     }
 
-    /// Favourites that sell the active fuel, cheapest first (distance tiebreak).
+    /// Stations favourited for the active fuel, cheapest first (distance tiebreak).
     private var ranked: [FuelStation] {
-        let available = favourites.filter { $0.prices[activeFuel] != nil }
+        let available = favourites.filter { $0.fuel == activeFuel }.map(\.station)
         return available.sorted { lhs, rhs in
             let lPrice = lhs.prices[activeFuel]!
             let rPrice = rhs.prices[activeFuel]!
@@ -41,20 +42,23 @@ struct FavouritesView: View {
         }
     }
 
+    /// IDs favourited for the active fuel — the star state on each row.
+    private var activeFuelFavouriteIDs: Set<String> {
+        Set(favourites.filter { $0.fuel == activeFuel }.map(\.station.id))
+    }
+
     private var cheapest: FuelStation? { ranked.first }
     private var cheapestPrice: Double? { cheapest?.prices[activeFuel] }
 
-    init(favourites: [FuelStation],
+    init(favourites: [FavouriteEntry],
          selectedFuel: FuelType,
          location: Coordinate?,
          distanceUnit: DistanceUnit,
-         favouriteIDs: Set<String>,
-         onToggleFavourite: @escaping (FuelStation) -> Void = { _ in }) {
+         onToggleFavourite: @escaping (FuelStation, FuelType) -> Void = { _, _ in }) {
         self.favourites = favourites
         self.selectedFuel = selectedFuel
         self.location = location
         self.distanceUnit = distanceUnit
-        self.favouriteIDs = favouriteIDs
         self.onToggleFavourite = onToggleFavourite
         _fuel = State(initialValue: selectedFuel)
     }
@@ -70,7 +74,7 @@ struct FavouritesView: View {
                                 .foregroundStyle(.secondary)
                             Text("No favourites yet")
                                 .font(.headline)
-                            Text("Tap the star on any station in the Stations tab to pin it here, ranked by which is cheapest.")
+                            Text("Tap the star on any station in the Stations tab to pin it here, ranked by which is cheapest for that fuel.")
                                 .font(.caption)
                                 .foregroundStyle(.secondary)
                                 .multilineTextAlignment(.center)
@@ -108,8 +112,8 @@ struct FavouritesView: View {
                                 distanceUnit: distanceUnit,
                                 baselinePrice: cheapestPrice,
                                 isTopResult: index == 0,
-                                isFavourite: favouriteIDs.contains(station.id),
-                                onToggleFavourite: { onToggleFavourite(station) }
+                                isFavourite: activeFuelFavouriteIDs.contains(station.id),
+                                onToggleFavourite: { onToggleFavourite(station, activeFuel) }
                             )
                         }
                     }