feat(03-01): create OverlayService and add retry dependency

- Add retry package (^3.1.2) for robust platform operations
- Create OverlayService wrapper for all FlutterOverlayWindow calls
- Implement showOverlay/closeOverlay with 3-attempt retry logic
- Add shareData fire-and-forget with warn-level logging
- Add isActive, overlayListener, checkPermission helpers
- Define staleness constants: 3s dim, 10s dash, 0.5 opacity, 1s check
- Define GPS_GRACE_PERIOD constant (30 seconds)

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

lib/services/overlay_service.dart

@@ -0,0 +1,96 @@
+import 'package:flutter_overlay_window/flutter_overlay_window.dart';
+import 'package:permission_handler/permission_handler.dart';
+import 'package:retry/retry.dart';
+import 'logger.dart';
+
+/// Service wrapper for FlutterOverlayWindow platform calls with retry logic
+/// Encapsulates all overlay operations with error handling and verification
+class OverlayService {
+  /// Retry configuration for platform operations
+  static final RetryOptions _retryOptions = RetryOptions(
+    maxAttempts: 3,
+    delayFactor: const Duration(milliseconds: 200),
+  );
+
+  /// Show overlay window with retry logic and verification
+  /// Returns true if overlay successfully opened and verified active
+  Future<bool> showOverlay({
+    required int width,
+    required int height,
+  }) async {
+    try {
+      await _retryOptions.retry(
+        () async {
+          await FlutterOverlayWindow.showOverlay(
+            enableDrag: true,
+            overlayTitle: "Speedometer",
+            overlayContent: 'Speedo overlay active',
+            flag: OverlayFlag.defaultFlag,
+            visibility: NotificationVisibility.visibilityPublic,
+            positionGravity: PositionGravity.none,
+            width: width,
+            height: height,
+          );
+        },
+      );
+
+      // Verify overlay actually opened
+      final isActive = await FlutterOverlayWindow.isActive();
+      if (!isActive) {
+        Logger.error('Overlay showOverlay succeeded but isActive returned false', 'OverlayService');
+        return false;
+      }
+
+      return true;
+    } catch (e) {
+      Logger.error('Failed to show overlay after retries: $e', 'OverlayService');
+      return false;
+    }
+  }
+
+  /// Close overlay window with retry logic
+  /// Returns true on success, false on failure after retries
+  Future<bool> closeOverlay() async {
+    try {
+      await _retryOptions.retry(
+        () async {
+          await FlutterOverlayWindow.closeOverlay();
+        },
+      );
+      return true;
+    } catch (e) {
+      Logger.error('Failed to close overlay after retries: $e', 'OverlayService');
+      return false;
+    }
+  }
+
+  /// Share data with overlay window (fire-and-forget)
+  /// Does NOT retry - data loss is expected and handled by staleness detection
+  Future<void> shareData(Map<String, dynamic> data) async {
+    try {
+      await FlutterOverlayWindow.shareData(data);
+    } catch (e) {
+      Logger.warn('Failed to share data with overlay: $e', 'OverlayService');
+    }
+  }
+
+  /// Check if overlay window is currently active
+  /// Returns false on error
+  Future<bool> isActive() async {
+    try {
+      return await FlutterOverlayWindow.isActive();
+    } catch (e) {
+      Logger.error('Failed to check overlay active status: $e', 'OverlayService');
+      return false;
+    }
+  }
+
+  /// Stream of messages from overlay window
+  /// Direct delegation to platform channel
+  Stream<dynamic> get overlayListener => FlutterOverlayWindow.overlayListener;
+
+  /// Check if system alert window permission is granted
+  Future<bool> checkPermission() async {
+    return await Permission.systemAlertWindow.isGranted;
+  }
+}