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.

FuelBoard/RoadDistanceService.swift

@@ -15,17 +15,22 @@ import MapKit
 import WidgetKit
 
 enum RoadDistanceService {
-    /// How many nearest stations to route per pass (bounds MKDirections calls).
-    static let candidatesPerPass = 12
+    /// Upper bound on stations routed per pass, so a pass stays a bounded set of
+    /// route calls. Raised from 12 so stations past the old nearest-12 cutoff
+    /// still get real road distances instead of a straight-line fallback.
+    static let candidatesPerPass = 40
+    /// Only route stations within this straight-line radius (km). Covers the
+    /// largest search radius the UI exposes (15 mi ≈ 24.1 km) plus margin, so
+    /// every station a widget/Live Activity/list can actually show gets routed.
+    static let maxRadiusKM: Double = 25
     /// 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.
+    /// Refreshes the cached road distances for the in-radius stations around
+    /// `lat`/`lng`. Throttled by time + distance; safe to call on every fix.
     static func refreshIfNeeded(stations: [FuelStation], lat: Double, lng: Double) async {
         guard !stations.isEmpty else { return }
 
@@ -38,11 +43,12 @@ enum RoadDistanceService {
             }
         }
 
-        // Candidate stations: nearest by straight-line (these are what widgets
-        // and the Live Activity show in-radius).
+        // Candidate stations: the nearest-by-straight-line subset that the UI
+        // could actually display, capped so a pass stays bounded.
         let nearest = stations
             .sorted { $0.distanceKM(to: lat, lng2: lng) < $1.distanceKM(to: lat, lng2: lng) }
             .prefix(candidatesPerPass)
+            .filter { $0.distanceKM(to: lat, lng2: lng) <= maxRadiusKM }
 
         let origin = CLLocationCoordinate2D(latitude: lat, longitude: lng)
         var entries: [String: Double] = [:]