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.

Shared/FuelStore.swift

@@ -275,6 +275,73 @@ struct FuelStation: Identifiable, Codable, Equatable {
     let prices: [FuelType: Double]
     /// Updated timestamp (seconds since 1970) — nil for sample data.
     let priceUpdated: TimeInterval?
+    /// Per-grade timestamps when the source provides them. A station-wide
+    /// `price_updated` must not be copied into this map: it does not prove that
+    /// every grade changed at that time.
+    let priceUpdatedByFuel: [FuelType: TimeInterval]?
+    /// Original per-grade timestamp text, retained so malformed source values
+    /// cannot silently fall back to a station-wide timestamp.
+    let priceUpdatedByFuelRaw: [FuelType: String]?
+
+    init(id: String, name: String, brand: String, address: String, postcode: String,
+         lat: Double, lng: Double, prices: [FuelType: Double],
+         priceUpdated: TimeInterval?, priceUpdatedByFuel: [FuelType: TimeInterval]? = nil,
+         priceUpdatedByFuelRaw: [FuelType: String]? = nil) {
+        self.id = id
+        self.name = name
+        self.brand = brand
+        self.address = address
+        self.postcode = postcode
+        self.lat = lat
+        self.lng = lng
+        self.prices = prices
+        self.priceUpdated = priceUpdated
+        self.priceUpdatedByFuel = priceUpdatedByFuel
+        self.priceUpdatedByFuelRaw = priceUpdatedByFuelRaw
+    }
+
+    private enum CodingKeys: String, CodingKey {
+        case id, name, brand, address, postcode, lat, lng, prices
+        case priceUpdated = "priceUpdated"
+        case priceUpdatedSnake = "price_updated"
+        case priceUpdatedByFuel = "priceUpdatedByFuel"
+        case priceUpdatedByFuelSnake = "price_updated_by_fuel"
+        case priceUpdatedByFuelRaw = "priceUpdatedByFuelRaw"
+        case priceUpdatedByFuelRawSnake = "price_updated_by_fuel_raw"
+    }
+
+    init(from decoder: Decoder) throws {
+        let values = try decoder.container(keyedBy: CodingKeys.self)
+        id = try values.decode(String.self, forKey: .id)
+        name = try values.decode(String.self, forKey: .name)
+        brand = try values.decode(String.self, forKey: .brand)
+        address = try values.decode(String.self, forKey: .address)
+        postcode = try values.decode(String.self, forKey: .postcode)
+        lat = try values.decode(Double.self, forKey: .lat)
+        lng = try values.decode(Double.self, forKey: .lng)
+        prices = try values.decode([FuelType: Double].self, forKey: .prices)
+        priceUpdated = try values.decodeIfPresent(TimeInterval.self, forKey: .priceUpdated)
+            ?? values.decodeIfPresent(TimeInterval.self, forKey: .priceUpdatedSnake)
+        priceUpdatedByFuel = try values.decodeIfPresent([FuelType: TimeInterval].self, forKey: .priceUpdatedByFuel)
+            ?? values.decodeIfPresent([FuelType: TimeInterval].self, forKey: .priceUpdatedByFuelSnake)
+        priceUpdatedByFuelRaw = try values.decodeIfPresent([FuelType: String].self, forKey: .priceUpdatedByFuelRaw)
+            ?? values.decodeIfPresent([FuelType: String].self, forKey: .priceUpdatedByFuelRawSnake)
+    }
+
+    func encode(to encoder: Encoder) throws {
+        var values = encoder.container(keyedBy: CodingKeys.self)
+        try values.encode(id, forKey: .id)
+        try values.encode(name, forKey: .name)
+        try values.encode(brand, forKey: .brand)
+        try values.encode(address, forKey: .address)
+        try values.encode(postcode, forKey: .postcode)
+        try values.encode(lat, forKey: .lat)
+        try values.encode(lng, forKey: .lng)
+        try values.encode(prices, forKey: .prices)
+        try values.encodeIfPresent(priceUpdated, forKey: .priceUpdated)
+        try values.encodeIfPresent(priceUpdatedByFuel, forKey: .priceUpdatedByFuel)
+        try values.encodeIfPresent(priceUpdatedByFuelRaw, forKey: .priceUpdatedByFuelRaw)
+    }
 
     /// Cheap haversine distance to a location, in km.
     func distanceKM(to lat2: Double, lng2: Double) -> Double {