refactor(bridge): move platform_bridge helpers into part files

Top-level helpers and secondary classes (codec isolates, cache entries,
event and SAF models, the native worker job state and the NDJSON scan
file) move verbatim into part files grouped by concern. The
PlatformBridge class body is unchanged apart from three region comments
over groups that were already contiguous.

platform_bridge.dart was one 2900-line file mixing eight concerns; the
static class cannot be split without changing its type, so this moves
what can move mechanically and leaves the class intact for call sites
and the channel mocks in tests.

lib/services/platform_bridge.dart

@@ -9,6 +9,13 @@ import 'package:spotiflac_android/services/download_request_payload.dart';
 import 'package:spotiflac_android/utils/audio_format_utils.dart';
 import 'package:spotiflac_android/utils/logger.dart';
 
+part 'platform_bridge/bridge_codec.dart';
+part 'platform_bridge/bridge_events.dart';
+part 'platform_bridge/bridge_library_scan.dart';
+part 'platform_bridge/bridge_lookup_cache.dart';
+part 'platform_bridge/bridge_native_worker.dart';
+part 'platform_bridge/bridge_saf.dart';
+
 final _log = AppLogger('PlatformBridge');
 
 const foregroundServiceStartNotAllowedCode =
@@ -19,119 +26,6 @@ bool isForegroundServiceStartNotAllowed(Object error) {
       error.code == foregroundServiceStartNotAllowedCode;
 }
 
-Object? _decodeJsonInBackground(String json) => jsonDecode(json);
-String _encodeJsonInBackground(Object? value) => jsonEncode(value);
-
-class _NativeDataJobState {
-  bool dispatched = false;
-  bool cancelled = false;
-}
-
-String _encodeBoundedLookupCacheInBackground(Map<String, dynamic> entries) {
-  final result = StringBuffer('{');
-  var bytes = 2; // Opening and closing braces.
-  var count = 0;
-  for (final entry in entries.entries) {
-    final fragment =
-        '${_encodeJsonInBackground(entry.key)}:${_encodeJsonInBackground(entry.value)}';
-    final entryBytes = utf8.encode(fragment).length;
-    final separatorBytes = count == 0 ? 0 : 1;
-    if (entryBytes > PlatformBridge._persistentCacheMaxEntryBytes ||
-        bytes + separatorBytes + entryBytes >
-            PlatformBridge._persistentCacheMaxBytes) {
-      continue;
-    }
-    if (count > 0) result.write(',');
-    result.write(fragment);
-    bytes += separatorBytes + entryBytes;
-    count++;
-  }
-  result.write('}');
-  return result.toString();
-}
-
-Object? _decodeJsonFileInBackground(String path) {
-  final contents = File(path).readAsStringSync();
-  return contents.isEmpty ? null : jsonDecode(contents);
-}
-
-class LibraryScanNDJSONFile {
-  final File file;
-  final int expectedCount;
-  final int errorCount;
-
-  const LibraryScanNDJSONFile({
-    required this.file,
-    required this.expectedCount,
-    this.errorCount = 0,
-  });
-
-  Stream<Map<String, dynamic>> rows() async* {
-    await for (final line
-        in file
-            .openRead()
-            .transform(utf8.decoder)
-            .transform(const LineSplitter())) {
-      if (line.trim().isEmpty) continue;
-      final decoded = jsonDecode(line);
-      if (decoded is! Map) {
-        throw const FormatException('Library scan NDJSON row is not an object');
-      }
-      yield Map<String, dynamic>.from(decoded);
-    }
-  }
-
-  Future<void> delete() async {
-    try {
-      if (await file.exists()) await file.delete();
-    } catch (_) {}
-    // Remove the resumable scan checkpoint after ingestion.
-    try {
-      final checkpoint = File('${file.path}.state');
-      if (await checkpoint.exists()) await checkpoint.delete();
-    } catch (_) {}
-  }
-}
-
-class ExtensionSessionGrantEvent {
-  final String extensionId;
-  final bool success;
-
-  const ExtensionSessionGrantEvent({
-    required this.extensionId,
-    required this.success,
-  });
-}
-
-/// Folder picked via the native iOS document picker: the filesystem path plus
-/// the security-scoped bookmark that re-grants access across app launches.
-class IosPickedDirectory {
-  final String path;
-  final String bookmark;
-
-  const IosPickedDirectory({required this.path, required this.bookmark});
-}
-
-class IosSecurityScopedAccess {
-  final String path;
-  final String token;
-
-  const IosSecurityScopedAccess({required this.path, required this.token});
-}
-
-class ContentUriPlaybackLease {
-  final String path;
-  final String token;
-
-  final String? displayName;
-
-  const ContentUriPlaybackLease({
-    required this.path,
-    required this.token,
-    this.displayName,
-  });
-}
-
 class InstallationState {
   final bool markerExisted;
   final bool markerCreated;
@@ -167,46 +61,6 @@ bool shouldResetRestoredInstallation({
       installState.freshPackageInstall;
 }
 
-class _BridgeCacheEntry {
-  final Map<String, dynamic> value;
-  final DateTime expiresAt;
-  final int bytes;
-
-  const _BridgeCacheEntry({
-    required this.value,
-    required this.expiresAt,
-    required this.bytes,
-  });
-
-  bool get isExpired => DateTime.now().isAfter(expiresAt);
-}
-
-class _BridgeListCacheEntry {
-  final List<Map<String, dynamic>> value;
-  final DateTime expiresAt;
-  final int bytes;
-
-  const _BridgeListCacheEntry({
-    required this.value,
-    required this.expiresAt,
-    required this.bytes,
-  });
-
-  bool get isExpired => DateTime.now().isAfter(expiresAt);
-}
-
-class _BridgeInFlight<T> {
-  final String requestId;
-  final String scopeKey;
-  final Future<T> future;
-
-  const _BridgeInFlight({
-    required this.requestId,
-    required this.scopeKey,
-    required this.future,
-  });
-}
-
 class PlatformBridge {
   /// Sentinel shared with Android's foreground service for progress reports
   /// that contain a percentage but no byte counts.
@@ -1022,6 +876,8 @@ class PlatformBridge {
     return result as String;
   }
 
+  // ---- SAF ----
+
   static Future<Map<String, dynamic>?> pickSafTree({
     bool forSetup = false,
   }) async {
@@ -1262,6 +1118,8 @@ class PlatformBridge {
     return result as bool? ?? false;
   }
 
+  // ---- Media files ----
+
   static Future<String> getLyricsLRC(
     String spotifyId,
     String trackName,
@@ -2396,6 +2254,8 @@ class PlatformBridge {
     }
   }
 
+  // ---- Library scan ----
+
   static Future<void> setLibraryCoverCacheDir(String cacheDir) async {
     await _channel.invokeMethod('setLibraryCoverCacheDir', {
       'cache_dir': cacheDir,

lib/services/platform_bridge/bridge_codec.dart

@@ -0,0 +1,32 @@
+part of '../platform_bridge.dart';
+
+Object? _decodeJsonInBackground(String json) => jsonDecode(json);
+String _encodeJsonInBackground(Object? value) => jsonEncode(value);
+
+String _encodeBoundedLookupCacheInBackground(Map<String, dynamic> entries) {
+  final result = StringBuffer('{');
+  var bytes = 2; // Opening and closing braces.
+  var count = 0;
+  for (final entry in entries.entries) {
+    final fragment =
+        '${_encodeJsonInBackground(entry.key)}:${_encodeJsonInBackground(entry.value)}';
+    final entryBytes = utf8.encode(fragment).length;
+    final separatorBytes = count == 0 ? 0 : 1;
+    if (entryBytes > PlatformBridge._persistentCacheMaxEntryBytes ||
+        bytes + separatorBytes + entryBytes >
+            PlatformBridge._persistentCacheMaxBytes) {
+      continue;
+    }
+    if (count > 0) result.write(',');
+    result.write(fragment);
+    bytes += separatorBytes + entryBytes;
+    count++;
+  }
+  result.write('}');
+  return result.toString();
+}
+
+Object? _decodeJsonFileInBackground(String path) {
+  final contents = File(path).readAsStringSync();
+  return contents.isEmpty ? null : jsonDecode(contents);
+}

lib/services/platform_bridge/bridge_events.dart

@@ -0,0 +1,11 @@
+part of '../platform_bridge.dart';
+
+class ExtensionSessionGrantEvent {
+  final String extensionId;
+  final bool success;
+
+  const ExtensionSessionGrantEvent({
+    required this.extensionId,
+    required this.success,
+  });
+}

lib/services/platform_bridge/bridge_library_scan.dart

@@ -0,0 +1,39 @@
+part of '../platform_bridge.dart';
+
+class LibraryScanNDJSONFile {
+  final File file;
+  final int expectedCount;
+  final int errorCount;
+
+  const LibraryScanNDJSONFile({
+    required this.file,
+    required this.expectedCount,
+    this.errorCount = 0,
+  });
+
+  Stream<Map<String, dynamic>> rows() async* {
+    await for (final line
+        in file
+            .openRead()
+            .transform(utf8.decoder)
+            .transform(const LineSplitter())) {
+      if (line.trim().isEmpty) continue;
+      final decoded = jsonDecode(line);
+      if (decoded is! Map) {
+        throw const FormatException('Library scan NDJSON row is not an object');
+      }
+      yield Map<String, dynamic>.from(decoded);
+    }
+  }
+
+  Future<void> delete() async {
+    try {
+      if (await file.exists()) await file.delete();
+    } catch (_) {}
+    // Remove the resumable scan checkpoint after ingestion.
+    try {
+      final checkpoint = File('${file.path}.state');
+      if (await checkpoint.exists()) await checkpoint.delete();
+    } catch (_) {}
+  }
+}

lib/services/platform_bridge/bridge_lookup_cache.dart

@@ -0,0 +1,41 @@
+part of '../platform_bridge.dart';
+
+class _BridgeCacheEntry {
+  final Map<String, dynamic> value;
+  final DateTime expiresAt;
+  final int bytes;
+
+  const _BridgeCacheEntry({
+    required this.value,
+    required this.expiresAt,
+    required this.bytes,
+  });
+
+  bool get isExpired => DateTime.now().isAfter(expiresAt);
+}
+
+class _BridgeListCacheEntry {
+  final List<Map<String, dynamic>> value;
+  final DateTime expiresAt;
+  final int bytes;
+
+  const _BridgeListCacheEntry({
+    required this.value,
+    required this.expiresAt,
+    required this.bytes,
+  });
+
+  bool get isExpired => DateTime.now().isAfter(expiresAt);
+}
+
+class _BridgeInFlight<T> {
+  final String requestId;
+  final String scopeKey;
+  final Future<T> future;
+
+  const _BridgeInFlight({
+    required this.requestId,
+    required this.scopeKey,
+    required this.future,
+  });
+}

lib/services/platform_bridge/bridge_native_worker.dart

@@ -0,0 +1,6 @@
+part of '../platform_bridge.dart';
+
+class _NativeDataJobState {
+  bool dispatched = false;
+  bool cancelled = false;
+}

lib/services/platform_bridge/bridge_saf.dart

@@ -0,0 +1,30 @@
+part of '../platform_bridge.dart';
+
+/// Folder picked via the native iOS document picker: the filesystem path plus
+/// the security-scoped bookmark that re-grants access across app launches.
+class IosPickedDirectory {
+  final String path;
+  final String bookmark;
+
+  const IosPickedDirectory({required this.path, required this.bookmark});
+}
+
+class IosSecurityScopedAccess {
+  final String path;
+  final String token;
+
+  const IosSecurityScopedAccess({required this.path, required this.token});
+}
+
+class ContentUriPlaybackLease {
+  final String path;
+  final String token;
+
+  final String? displayName;
+
+  const ContentUriPlaybackLease({
+    required this.path,
+    required this.token,
+    this.displayName,
+  });
+}