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/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,
     );
   }
 }