fix(library): preserve partial scans and ignore stale completion events

test/library_full_scan_test.dart

@@ -123,7 +123,7 @@ void main() {
 
     for (final count in [0, 2]) {
       test(
-        'partial scan cannot replace the index ($count rows, SAF=$saf)',
+        'partial scan exposes readable rows and errors ($count rows, SAF=$saf)',
         () async {
           messenger.setMockMethodCallHandler(backend, (call) async {
             final output = File(
@@ -132,16 +132,10 @@ void main() {
             await output.writeAsString('{"id":"readable"}\n' * count);
             return {'path': output.path, 'count': count, 'error_count': 3};
           });
-          await expectLater(
-            scan(),
-            throwsA(
-              isA<FormatException>().having(
-                (error) => error.message,
-                'reason',
-                contains('existing Library was kept'),
-              ),
-            ),
-          );
+          final partial = await scan();
+          expect(partial.errorCount, 3);
+          expect(partial.expectedCount, count);
+          expect(await partial.rows().toList(), hasLength(count));
         },
       );
     }

test/library_partial_scan_test.dart

@@ -0,0 +1,77 @@
+import 'dart:convert';
+import 'dart:io';
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:spotiflac_android/services/library_cleanup.dart';
+import 'package:sqflite/sqflite.dart';
+
+class _Statements implements DatabaseExecutor {
+  final statements = <Map<String, Object?>>[];
+
+  @override
+  Future<int> rawDelete(String sql, [List<Object?>? arguments]) async {
+    statements.add({'sql': sql, 'args': arguments});
+    return 0;
+  }
+
+  @override
+  dynamic noSuchMethod(Invocation invocation) => super.noSuchMethod(invocation);
+}
+
+void main() {
+  for (final partial in [false, true]) {
+    for (final empty in [false, true]) {
+      test(
+        'scan replacement preserves unreadable rows only on partial scans ($partial, empty=$empty)',
+        () async {
+          final db = _Statements();
+          await deleteReplacedLibraryScanRows(
+            db,
+            'sd-card',
+            stageTable: 'staged',
+            preserveMissing: partial,
+          );
+          // Run the actual generated SQL in SQLite, including path-key cleanup.
+          final result = await Process.run('python3', [
+            '-c',
+            r'''
+import json, sqlite3, sys
+args = json.loads(sys.argv[1])
+db = sqlite3.connect(':memory:')
+db.executescript("""
+CREATE TABLE library(id TEXT PRIMARY KEY, file_path TEXT UNIQUE, source_id TEXT);
+CREATE TABLE library_path_keys(item_id TEXT, path_key TEXT);
+CREATE TABLE staged(id TEXT, file_path TEXT);
+INSERT INTO library VALUES ('readable', '/sd/readable.flac', 'sd-card'),
+ ('old-id', '/sd/changed.flac', 'sd-card'),
+ ('unreadable', '/sd/unreadable.flac', 'sd-card'),
+ ('other', '/internal/track.flac', 'internal');
+INSERT INTO library_path_keys SELECT id, file_path FROM library;
+""")
+if not args['empty']:
+    db.executescript("INSERT INTO staged VALUES ('readable', '/sd/readable.flac'), ('new-id', '/sd/changed.flac');")
+with db:
+    for statement in args['statements']:
+        db.execute(statement['sql'], statement['args'])
+print(json.dumps({
+    'rows': [row[0] for row in db.execute('SELECT id FROM library ORDER BY id')],
+    'keys': [row[0] for row in db.execute('SELECT item_id FROM library_path_keys ORDER BY item_id')]
+}))
+''',
+            jsonEncode({'statements': db.statements, 'empty': empty}),
+          ]);
+          expect(result.exitCode, 0, reason: result.stderr.toString());
+          final data =
+              jsonDecode(result.stdout as String) as Map<String, dynamic>;
+          final expected = partial
+              ? (empty
+                    ? ['old-id', 'other', 'readable', 'unreadable']
+                    : ['other', 'unreadable'])
+              : ['other'];
+          expect(data['rows'], expected);
+          expect(data['keys'], expected);
+        },
+      );
+    }
+  }
+}