perf(library): stream normalized scan timestamp snapshots

lib/providers/local_library_provider.dart

@@ -759,7 +759,7 @@ class LocalLibraryNotifier extends Notifier<LocalLibraryState> {
         final useSnapshotBridge =
             Platform.isAndroid && existingFiles.isNotEmpty;
         final snapshotPath = useSnapshotBridge
-            ? await _db.writeFileModTimesSnapshot(sourceId: activeSourceId)
+            ? await _db.writeFileModTimesSnapshot(existingFiles)
             : null;
 
         Map<String, dynamic> result;
@@ -1263,7 +1263,9 @@ class LocalLibraryNotifier extends Notifier<LocalLibraryState> {
     required Map<String, int> existingFiles,
   }) async {
     final legacyPaths = existingFiles.entries
-        .where((entry) => entry.value <= 0)
+        // Negative timestamps deliberately force a metadata-version rescan;
+        // only a missing timestamp (zero) may be filled from the filesystem.
+        .where((entry) => entry.value == 0)
         .map((entry) => entry.key)
         .toList();
     if (legacyPaths.isEmpty) {

lib/services/library_database.dart

@@ -2064,14 +2064,11 @@ class LibraryDatabase {
     return result;
   }
 
-  Future<String> writeFileModTimesSnapshot({String? sourceId}) async {
-    final db = await database;
-    final rows = await db.rawQuery(
-      'SELECT file_path, COALESCE(file_mod_time, 0) AS file_mod_time, '
-      'audio_metadata_scan_version FROM library '
-      '${sourceId == null ? '' : 'WHERE source_id = ?'}',
-      sourceId == null ? const [] : [sourceId],
-    );
+  /// Uses the already normalized, backfilled timestamps from the scan caller.
+  /// Bounded writes avoid a second query and a library-sized StringBuffer.
+  Future<String> writeFileModTimesSnapshot(
+    Map<String, int> fileModTimes,
+  ) async {
     final tempDir = await getTemporaryDirectory();
     final file = File(
       join(
@@ -2079,24 +2076,30 @@ class LibraryDatabase {
         'library_file_mod_times_${DateTime.now().microsecondsSinceEpoch}.tsv',
       ),
     );
-    final buffer = StringBuffer();
-    for (final row in rows) {
-      final path = row['file_path'] as String?;
-      if (path == null || path.isEmpty) continue;
-      final modTime = (row['file_mod_time'] as num?)?.toInt() ?? 0;
-      final scanVersion =
-          (row['audio_metadata_scan_version'] as num?)?.toInt() ?? 0;
-      buffer
-        ..write(
-          libraryIncrementalSnapshotModTime(
-            storedModTime: modTime,
-            storedScanVersion: scanVersion,
-          ),
-        )
-        ..write('\t')
-        ..writeln(path);
+    try {
+      final output = await file.open(mode: FileMode.write);
+      try {
+        final buffer = StringBuffer();
+        for (final entry in fileModTimes.entries) {
+          if (entry.key.isEmpty) continue;
+          buffer
+            ..write(entry.value)
+            ..write('\t')
+            ..writeln(entry.key);
+          if (buffer.length >= 64 * 1024) {
+            await output.writeString(buffer.toString());
+            buffer.clear();
+          }
+        }
+        if (buffer.isNotEmpty) await output.writeString(buffer.toString());
+        await output.flush();
+      } finally {
+        await output.close();
+      }
+    } catch (_) {
+      if (await file.exists()) await file.delete();
+      rethrow;
     }
-    await file.writeAsString(buffer.toString(), flush: true);
     return file.path;
   }
 

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(),
+      );
+    },
+  );
+}