perf(library): stream normalized scan timestamp snapshots

test/library_scan_snapshot_test.dart

@@ -0,0 +1,39 @@
+import 'dart:io';
+import 'package:flutter/services.dart';
+import 'package:spotiflac_android/services/library_database.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  TestWidgetsFlutterBinding.ensureInitialized();
+  test(
+    'scan snapshot preserves normalized sentinel and backfilled timestamps across chunks',
+    () async {
+      final directory = await Directory.systemTemp.createTemp(
+        'scan-snapshot-test-',
+      );
+      const channel = MethodChannel('plugins.flutter.io/path_provider');
+      final messenger =
+          TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger;
+      messenger.setMockMethodCallHandler(channel, (_) async => directory.path);
+      addTearDown(() async {
+        messenger.setMockMethodCallHandler(channel, null);
+        await directory.delete(recursive: true);
+      });
+      final timestamps = <String, int>{
+        for (var i = 0; i < 4000; i++) '/music/歌曲 café $i.flac': i,
+        '/music/legacy.flac': -1,
+        '/music/backfilled.flac': 1720000000000,
+      };
+      final path = await LibraryDatabase.instance.writeFileModTimesSnapshot(
+        timestamps,
+      );
+      final lines = await File(path).readAsLines();
+      expect(
+        lines,
+        timestamps.entries
+            .map((entry) => '${entry.value}\t${entry.key}')
+            .toList(),
+      );
+    },
+  );
+}