feat(04-01): add timestamp to ProcessedGpsData and precision/lifecycle constants

- Add timestamp field to ProcessedGpsData with copyWith support
- Add precision switching thresholds: 3.33 m/s (12 km/h) up, 2.22 m/s (8 km/h) down
- Add PRECISION_SWITCH_FIRST_FIX_DELAY (5s) to prevent early switching
- Add BACKGROUND_GRACE_PERIOD (7s) for lifecycle-aware GPS management
- Mark GPS_GRACE_PERIOD as deprecated in favor of BACKGROUND_GRACE_PERIOD
- Change GpsService.positionStream to createPositionStream() with dynamic accuracy
- Keep deprecated positionStream getter for backward compatibility

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

lib/gps_service.dart

@@ -6,15 +6,24 @@ import 'config/gps_constants.dart';
 class GpsService {
   static double? _lastValidHeading;
 
-  static Stream<Position> get positionStream {
+  /// Create a position stream with dynamic accuracy setting
+  static Stream<Position> createPositionStream({
+    LocationAccuracy accuracy = LocationAccuracy.high,
+  }) {
     return Geolocator.getPositionStream(
-      locationSettings: const LocationSettings(
-        accuracy: LocationAccuracy.bestForNavigation,
+      locationSettings: LocationSettings(
+        accuracy: accuracy,
         distanceFilter: 0,
       ),
     );
   }
 
+  /// Deprecated: Use createPositionStream() instead
+  @Deprecated('Use createPositionStream(accuracy: LocationAccuracy.bestForNavigation) instead')
+  static Stream<Position> get positionStream {
+    return createPositionStream(accuracy: LocationAccuracy.bestForNavigation);
+  }
+
   static Future<bool> requestPermissions() async {
     final permission = await Permission.locationWhenInUse.request();
     return permission.isGranted;