feat(core): 设备号改用系统标识，重装不再变成新游客

- Android 读 SSAID、iOS 读 IDFV，均免权限，由原生通道提供，不引插件
- 取不到时回退本地 UUIDv4，替代原来 32 位随机 + 时间戳的拼接

test/core/device_id_test.dart

@@ -0,0 +1,65 @@
+// Author: fruit.com.cn
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:fruitshorts/core/data/services/auth_session_store.dart';
+import 'package:fruitshorts/core/data/services/device_id_provider.dart';
+import 'package:shared_preferences_platform_interface/in_memory_shared_preferences_async.dart';
+import 'package:shared_preferences_platform_interface/shared_preferences_async_platform_interface.dart';
+
+/// 可控的系统设备标识来源。
+class _FakeDeviceIdProvider implements DeviceIdProvider {
+  _FakeDeviceIdProvider(this.value);
+
+  final String? value;
+  int calls = 0;
+
+  @override
+  Future<String?> systemDeviceId() async {
+    calls++;
+    return value;
+  }
+}
+
+void main() {
+  setUp(() {
+    SharedPreferencesAsyncPlatform.instance =
+        InMemorySharedPreferencesAsync.empty();
+  });
+
+  test('should_prefer_system_device_id_over_local_value', () async {
+    final provider = _FakeDeviceIdProvider('a-9774d56d682e549c');
+    final store = AuthSessionStore(deviceIdProvider: provider);
+
+    expect(await store.deviceId(), 'a-9774d56d682e549c');
+  });
+
+  test('should_fall_back_to_stable_local_uuid_when_system_id_missing', () async {
+    final store = AuthSessionStore(
+      deviceIdProvider: _FakeDeviceIdProvider(null),
+    );
+
+    final first = await store.deviceId();
+    final second = await store.deviceId();
+
+    // 同一次运行内固定，且符合 UUIDv4 形态
+    expect(first, second);
+    expect(first, startsWith('d-'));
+    expect(
+      RegExp(
+        r'^d-[0-9a-f]{8}-[0-9a-f]{4}-4[0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}$',
+      ).hasMatch(first),
+      isTrue,
+      reason: '本地兜底必须是 UUIDv4，避免 32 位随机带来的碰撞',
+    );
+  });
+
+  test('should_cache_system_id_within_one_session', () async {
+    final provider = _FakeDeviceIdProvider('i-E621E1F8-C36C-495A-93FC-0C247A3E6E5F');
+    final store = AuthSessionStore(deviceIdProvider: provider);
+
+    await store.deviceId();
+    await store.deviceId();
+
+    expect(provider.calls, 1);
+  });
+}