refactor(utils): share the FNV-1a 32-bit hash

Add fnv1a32 in hash_utils.dart for the scan output name (UTF-8 bytes) and
qualityVariantStagingLabel (UTF-16 code units). Unit tests pin outputs
computed from the original inline code.

lib/services/platform_bridge.dart

@@ -8,6 +8,7 @@ import 'package:shared_preferences/shared_preferences.dart';
 import 'package:spotiflac_android/services/download_request_payload.dart';
 import 'package:spotiflac_android/utils/audio_format_utils.dart';
 import 'package:spotiflac_android/utils/file_access.dart';
+import 'package:spotiflac_android/utils/hash_utils.dart';
 import 'package:spotiflac_android/utils/logger.dart';
 
 part 'platform_bridge/bridge_codec.dart';
@@ -2326,11 +2327,7 @@ class PlatformBridge {
       'method': method,
       'arguments': arguments,
     });
-    var hash = 0x811c9dc5;
-    for (final byte in utf8.encode(identity)) {
-      hash ^= byte;
-      hash = (hash * 0x01000193) & 0xffffffff;
-    }
+    final hash = fnv1a32(utf8.encode(identity));
     final output = File(
       '${scanDir.path}${Platform.pathSeparator}'
       'library_scan_${hash.toRadixString(16).padLeft(8, '0')}.ndjson',

lib/utils/audio_format_utils.dart

@@ -1,3 +1,4 @@
+import 'package:spotiflac_android/utils/hash_utils.dart';
 import 'package:spotiflac_android/utils/int_utils.dart';
 import 'package:spotiflac_android/utils/string_utils.dart';
 
@@ -290,11 +291,7 @@ int? _bitrateFromQuality(String? quality) {
 }
 
 String qualityVariantStagingLabel(String itemId) {
-  var hash = 0x811c9dc5;
-  for (final byte in itemId.codeUnits) {
-    hash ^= byte;
-    hash = (hash * 0x01000193) & 0xffffffff;
-  }
+  final hash = fnv1a32(itemId.codeUnits);
   return 'qv_${hash.toRadixString(16).padLeft(8, '0')}';
 }
 

lib/utils/hash_utils.dart

@@ -0,0 +1,11 @@
+/// FNV-1a 32-bit hash over [units] (UTF-8 bytes or UTF-16 code units).
+///
+/// The result feeds on-disk file names, so the algorithm must not change.
+int fnv1a32(Iterable<int> units) {
+  var hash = 0x811c9dc5;
+  for (final unit in units) {
+    hash ^= unit;
+    hash = (hash * 0x01000193) & 0xffffffff;
+  }
+  return hash;
+}

test/fnv1a32_test.dart

@@ -0,0 +1,32 @@
+import 'dart:convert';
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:spotiflac_android/utils/audio_format_utils.dart';
+import 'package:spotiflac_android/utils/hash_utils.dart';
+
+String _hex(int hash) => hash.toRadixString(16).padLeft(8, '0');
+
+void main() {
+  // Expected values were computed from the original inline implementations
+  // (scan output name over UTF-8 bytes, staging label over UTF-16 units).
+  test('fnv1a32 over UTF-8 bytes matches the scan output name hash', () {
+    expect(_hex(fnv1a32(utf8.encode(''))), '811c9dc5');
+    expect(_hex(fnv1a32(utf8.encode('track-123'))), '8277aead');
+    expect(
+      _hex(
+        fnv1a32(
+          utf8.encode('{"method":"scan","arguments":{"path":"content://x/y"}}'),
+        ),
+      ),
+      '94bc90df',
+    );
+    expect(_hex(fnv1a32(utf8.encode('café 日本'))), 'c843754b');
+  });
+
+  test('qualityVariantStagingLabel matches the original code units hash', () {
+    expect(qualityVariantStagingLabel(''), 'qv_811c9dc5');
+    expect(qualityVariantStagingLabel('a'), 'qv_e40c292c');
+    expect(qualityVariantStagingLabel('track-123'), 'qv_8277aead');
+    expect(qualityVariantStagingLabel('café 日本'), 'qv_73a64e9d');
+  });
+}