road distance: cover all in-radius stations, not just nearest 12

Stations past the nearest-12 cutoff silently fell back to straight-line,
which explained the remaining app-vs-Apple-Maps gaps (e.g. 3.5 straight-line
vs 5.9 road). Raise the per-pass cap to 40 and route every station within a
25 km straight-line radius (covers the 15 mi max search window) so any station
the widget/Live Activity/list can show gets a real road distance.

Also harden the new cache tests against persistent keychain state across
invocations (reset in setUp) so they pass deterministically.

FuelBoardTests/Tests/FuelBoardSharedTests/FuelBoardTests.swift

@@ -624,6 +624,14 @@ final class OfflineDataLabelTests: XCTestCase {
 // MARK: - Road distance cache
 
 final class RoadDistanceCacheTests: XCTestCase {
+    override func setUp() {
+        super.setUp()
+        // Keychain persists across invocations, so a cache left by an earlier
+        // test or run would pollute these. Overwrite with an empty, far-away
+        // cache (source at (0,0)) so every test starts from a clean slate.
+        FuelStore.saveRoadDistances(sourceLat: 0, sourceLng: 0, entries: [:])
+    }
+
     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)