widget + live activity: prefer Apple-Maps road distance, computed by the app

The widget/Live Activity showed straight-line haversine distance (0.8 mi)
while Apple Maps routes 1.8 mi. Road routing is too heavy for the widget
execution + ~40-70/day refresh budget, so the APP now computes it:

- New RoadDistanceService (app target): for the nearest 12 stations each
  pass, calls MapKit MKDirections (free, no API key, matches Apple Maps)
  and caches metres keyed by station ID.
- Cache stored in KEYCHAIN (fuelboard.roadDistances) so the widget reads it
  even on free SideStore accounts with no app-group container; only valid
  within 600 m of the location it was built from.
- Throttled: recompute max every 10 min, or when the user moves > 400 m;
  wired into the location-update hook + location onChange.
- Widget face, app station rows + Live Activity show the cached road
  distance, falling back to straight-line when absent.
- 4 new cache tests (105 total).

FuelBoard/RoadDistanceService.swift

@@ -0,0 +1,87 @@
+// RoadDistanceService.swift — computes Apple-Maps-matched ROAD distances for
+// nearby stations and caches them (keychain) so widgets + Live Activity can
+// show real driving distance instead of straight-line haversine.
+//
+// Runs only in the APP: MKDirections is network-bound and the widget extension
+// has a tiny execution budget + a ~40-70/day refresh budget, so routing belongs
+// here, not in the widget. The widget/Live Activity just read the cache.
+//
+// Throttling: recompute at most every `throttleMinutes`, or when the user has
+// moved `moveThresholdMeters` from where the cache was built. Bounded to the
+// `candidatesPerPass` nearest stations so a pass stays a handful of route calls.
+
+import Foundation
+import MapKit
+import WidgetKit
+
+enum RoadDistanceService {
+    /// How many nearest stations to route per pass (bounds MKDirections calls).
+    static let candidatesPerPass = 12
+    /// Don't route again more often than this (minutes).
+    static let throttleMinutes: Double = 10
+    /// Recompute when the user moves more than this (metres) from the last
+    /// source location.
+    static let moveThresholdMeters: Double = 400
+
+    /// Refreshes the cached road distances for the nearest `candidatesPerPass`
+    /// stations around `lat`/`lng`. Throttled by time + distance; safe to call
+    /// on every location fix.
+    static func refreshIfNeeded(stations: [FuelStation], lat: Double, lng: Double) async {
+        guard !stations.isEmpty else { return }
+
+        // Throttle: keep cached values when fresh and the user hasn't moved far.
+        if let cache = FuelStore.loadRoadDistances() {
+            let elapsed = Date().timeIntervalSince1970 - cache.updatedAt
+            let movedMeters = haversineMeters(cache.sourceLat, cache.sourceLng, lat, lng)
+            if elapsed < throttleMinutes * 60 && movedMeters < moveThresholdMeters {
+                return
+            }
+        }
+
+        // Candidate stations: nearest by straight-line (these are what widgets
+        // and the Live Activity show in-radius).
+        let nearest = stations
+            .sorted { $0.distanceKM(to: lat, lng2: lng) < $1.distanceKM(to: lat, lng2: lng) }
+            .prefix(candidatesPerPass)
+
+        let origin = CLLocationCoordinate2D(latitude: lat, longitude: lng)
+        var entries: [String: Double] = [:]
+        for station in nearest {
+            let dest = CLLocationCoordinate2D(latitude: station.lat, longitude: station.lng)
+            if let meters = await roadMeters(from: origin, to: dest) {
+                entries[station.id] = meters
+            }
+        }
+        guard !entries.isEmpty else { return }
+
+        FuelStore.saveRoadDistances(sourceLat: lat, sourceLng: lng, entries: entries)
+        // Wake the widgets so the new road distances surface immediately.
+        WidgetCenter.shared.reloadAllTimelines()
+    }
+
+    /// Driving distance (metres) between two coordinates via Apple Maps routing.
+    private static func roadMeters(from: CLLocationCoordinate2D, to: CLLocationCoordinate2D) async -> Double? {
+        let request = MKDirections.Request()
+        request.source = MKMapItem(placemark: MKPlacemark(coordinate: from))
+        request.destination = MKMapItem(placemark: MKPlacemark(coordinate: to))
+        request.transportType = .automobile
+        request.requestsAlternateRoutes = false
+        do {
+            let response = try await MKDirections(request: request).calculate()
+            return response.routes.first?.distance
+        } catch {
+            return nil
+        }
+    }
+
+    /// Straight-line haversine distance between two coordinates, in metres.
+    private static func haversineMeters(_ lat1: Double, _ lng1: Double, _ lat2: Double, _ lng2: Double) -> Double {
+        let r = 6371000.0
+        let dLat = (lat2 - lat1) * .pi / 180
+        let dLng = (lng2 - lng1) * .pi / 180
+        let a = sin(dLat / 2) * sin(dLat / 2) +
+                cos(lat1 * .pi / 180) * cos(lat2 * .pi / 180) *
+                sin(dLng / 2) * sin(dLng / 2)
+        return r * 2 * atan2(sqrt(a), sqrt(1 - a))
+    }
+}