Add centralized retry/fallback policy with hard-coded endpoints

Extract duplicated retry logic from OverpassService and RoutingService
into a shared resilience framework in service_policy.dart:

- ResiliencePolicy: configurable retries, backoff, and HTTP timeout
- executeWithFallback: retry loop with primary→fallback endpoint chain
- ErrorDisposition enum: abort / fallback / retry classification
- ServicePolicy + ServicePolicyResolver: per-service compliance rules
  (rate limits, caching, concurrency) for OSMF and third-party services
- ServiceRateLimiter: async semaphore-based concurrency and rate control

OverpassService now hits overpass.deflock.org first, falls back to
overpass-api.de. RoutingService hits api.dontgetflocked.com first,
falls back to alprwatch.org. Both use per-service error classifiers
to determine retry vs fallback vs abort behavior.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/state/navigation_state.dart

@@ -250,11 +250,11 @@ class NavigationState extends ChangeNotifier {
     _calculateRoute();
   }
   
-  /// Calculate route using alprwatch
+  /// Calculate route via RoutingService (primary + fallback endpoints).
   void _calculateRoute() {
     if (_routeStart == null || _routeEnd == null) return;
 
-    debugPrint('[NavigationState] Calculating route with alprwatch...');
+    debugPrint('[NavigationState] Calculating route...');
     _isCalculating = true;
     _routingError = null;
     notifyListeners();
@@ -271,7 +271,7 @@ class NavigationState extends ChangeNotifier {
       _showingOverview = true;
       _provisionalPinLocation = null; // Hide provisional pin
       
-      debugPrint('[NavigationState] alprwatch route calculated: ${routeResult.toString()}');
+      debugPrint('[NavigationState] Route calculated: ${routeResult.toString()}');
       notifyListeners();
       
     }).catchError((error) {