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/config/gps_constants.dart

@@ -19,4 +19,13 @@ abstract class GpsConfig {
 
   /// Speed threshold below which heading is considered unreliable (m/s)
   static const double LOW_SPEED_THRESHOLD = 1.0;
+
+  /// Speed threshold for switching to HIGH precision (m/s) - 12 km/h
+  static const double PRECISION_UP_THRESHOLD_MPS = 3.33;
+
+  /// Speed threshold for switching to BALANCED precision (m/s) - 8 km/h
+  static const double PRECISION_DOWN_THRESHOLD_MPS = 2.22;
+
+  /// Minimum time after first fix before precision switching starts
+  static const Duration PRECISION_SWITCH_FIRST_FIX_DELAY = Duration(seconds: 5);
 }

lib/config/timing_constants.dart

@@ -15,5 +15,9 @@ abstract class TimingConfig {
   static const Duration LONG_PRESS_SIGNAL_DELAY = Duration(milliseconds: 50);
 
   /// GPS grace period - continues for 30s after overlay close when backgrounded
+  /// DEPRECATED: Replaced by BACKGROUND_GRACE_PERIOD (7s) for lifecycle management
   static const Duration GPS_GRACE_PERIOD = Duration(seconds: 30);
+
+  /// Background grace period - time before GPS stops when app backgrounded without overlay
+  static const Duration BACKGROUND_GRACE_PERIOD = Duration(seconds: 7);
 }

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;

lib/models/processed_gps_data.dart

@@ -5,6 +5,7 @@ class ProcessedGpsData {
   final String displayHeading;  // Formatted heading text
   final bool isSpeedValid;      // True if from recent GPS
   final bool isHeadingValid;    // True if from recent GPS
+  final DateTime timestamp;     // Timestamp for staleness detection
 
   const ProcessedGpsData({
     required this.speed,
@@ -13,6 +14,7 @@ class ProcessedGpsData {
     required this.displayHeading,
     required this.isSpeedValid,
     required this.isHeadingValid,
+    required this.timestamp,
   });
 
   ProcessedGpsData copyWith({
@@ -22,6 +24,7 @@ class ProcessedGpsData {
     String? displayHeading,
     bool? isSpeedValid,
     bool? isHeadingValid,
+    DateTime? timestamp,
   }) {
     return ProcessedGpsData(
       speed: speed ?? this.speed,
@@ -30,6 +33,7 @@ class ProcessedGpsData {
       displayHeading: displayHeading ?? this.displayHeading,
       isSpeedValid: isSpeedValid ?? this.isSpeedValid,
       isHeadingValid: isHeadingValid ?? this.isHeadingValid,
+      timestamp: timestamp ?? this.timestamp,
     );
   }
 }