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).

FuelBoardTests/Tests/FuelBoardSharedTests/FuelBoardTests.swift

@@ -620,3 +620,46 @@ final class OfflineDataLabelTests: XCTestCase {
         XCTAssertNil(FuelStore.offlineDataLabel(from: "not-a-date"))
     }
 }
+
+// MARK: - Road distance cache
+
+final class RoadDistanceCacheTests: XCTestCase {
+    private func station(_ id: String, _ lat: Double, _ lng: Double) -> FuelStation {
+        FuelStation(id: id, name: id, brand: "X", address: "", postcode: "",
+                    lat: lat, lng: lng, prices: [:], priceUpdated: nil)
+    }
+
+    func testDisplayDistanceFallsBackToStraightLineWhenNoCache() {
+        // London user, station ~ London -> no cache -> straight-line haversine.
+        let s = station("a", 51.5074, -0.1278)
+        let km = FuelStore.displayDistanceKM(station: s, userLat: 51.6, userLng: -0.1)
+        XCTAssertEqual(km, s.distanceKM(to: 51.6, lng2: -0.1), accuracy: 0.0001)
+    }
+
+    func testRoadDistanceUsedWhenCachedNear() {
+        let s = station("a", 51.5074, -0.1278)
+        // Cache a road distance of 3.2 km for this station from the user's fix.
+        FuelStore.saveRoadDistances(sourceLat: 51.6, sourceLng: -0.1, entries: ["a": 3200])
+        let km = FuelStore.displayDistanceKM(station: s, userLat: 51.6, userLng: -0.1)
+        XCTAssertEqual(km, 3.2, accuracy: 0.0001)
+    }
+
+    func testRoadDistanceNilWhenOriginFar() {
+        let s = station("a", 51.5074, -0.1278)
+        // Cache built in London, but the user is now ~200 km away -> stale.
+        FuelStore.saveRoadDistances(sourceLat: 51.5074, sourceLng: -0.1278, entries: ["a": 3200])
+        let meters = FuelStore.roadDistanceMeters(for: "a", userLat: 53.4808, userLng: -2.2426)
+        XCTAssertNil(meters)
+        // And display falls back to straight-line.
+        let km = FuelStore.displayDistanceKM(station: s, userLat: 53.4808, userLng: -2.2426)
+        XCTAssertEqual(km, s.distanceKM(to: 53.4808, lng2: -2.2426), accuracy: 0.0001)
+    }
+
+    func testRoadDistanceUsedForOtherStationNotFound() {
+        FuelStore.saveRoadDistances(sourceLat: 51.6, sourceLng: -0.1, entries: ["a": 3200])
+        // A station that isn't in the cache falls back to straight-line.
+        let s = station("z", 51.51, -0.13)
+        let km = FuelStore.displayDistanceKM(station: s, userLat: 51.6, userLng: -0.1)
+        XCTAssertEqual(km, s.distanceKM(to: 51.6, lng2: -0.1), accuracy: 0.0001)
+    }
+}