refactor(services): clarify misleading names and drop a no-op rethrow

Remove the try/catch that only rethrew in the library scan helper, rename
the history loader that schedules a microtask, and fix two comments.

lib/providers/download_history_provider.dart

@@ -118,11 +118,11 @@ class DownloadHistoryNotifier extends Notifier<DownloadHistoryState> {
   @override
   DownloadHistoryState build() {
     ref.onDispose(() => _indexBumpTimer?.cancel());
-    _loadFromDatabaseSync();
+    _scheduleLoadFromDatabase();
     return DownloadHistoryState();
   }
 
-  void _loadFromDatabaseSync() {
+  void _scheduleLoadFromDatabase() {
     if (_isLoaded) return;
     _isLoaded = true;
     Future.microtask(() async {

lib/services/history_database.dart

@@ -52,7 +52,8 @@ class HistoryBatchLookupRequest {
   const HistoryBatchLookupRequest(this.tracks);
 
   /// An immutable request whose hash is computed only once. Use for lists
-  /// retained across widget builds; the legacy constructor remains available.
+  /// retained across widget builds; the const constructor recomputes the hash
+  /// on every use.
   factory HistoryBatchLookupRequest.snapshot(
     Iterable<HistoryLookupRequest> tracks,
   ) = _HistoryBatchLookupSnapshot;
@@ -339,6 +340,8 @@ class HistoryDatabase {
     return (value ?? '').trim().toLowerCase();
   }
 
+  /// Returns '' for an empty track name, unlike [LibraryDatabase.matchKeyFor],
+  /// which returns '|artist' in that case.
   static String matchKeyFor(String? trackName, String? artistName) {
     final track = normalizeLookupText(trackName);
     if (track.isEmpty) return '';

lib/services/platform_bridge.dart

@@ -2479,68 +2479,64 @@ class PlatformBridge {
       '${scanDir.path}${Platform.pathSeparator}'
       'library_scan_${hash.toRadixString(16).padLeft(8, '0')}.ndjson',
     );
-    try {
-      if (isCancelled?.call() == true) {
-        throw StateError('Library scan cancelled before native scan');
-      }
-      if (forceFullScan) {
-        // An explicit rescan must not resume metadata from an earlier attempt.
-        for (final file in [output, File('${output.path}.state')]) {
-          if (await file.exists()) await file.delete();
-        }
-        // Preserve covers referenced by the current DB until ingestion succeeds.
-        // Fresh paths bypass native disk and Flutter image caches together.
-        final covers = Directory('${scanDir.path}/library_covers');
-        await covers.create(recursive: true);
-        final freshCovers = await covers.createTemp('full_scan_');
-        await setLibraryCoverCacheDir(freshCovers.path);
-      }
-      if (isCancelled?.call() == true) {
-        throw StateError('Library scan cancelled before native scan');
-      }
-      final result = await _channel.invokeMethod(method, {
-        ...arguments,
-        'output_path': output.path,
-      });
-      if (result is! Map) {
-        throw FormatException('$method returned ${result.runtimeType}');
-      }
-      if (result['cancelled'] == true) {
-        throw FormatException('$method returned a cancelled partial scan');
-      }
-      final errors = result['error_count'];
-      if (errors is! num ||
-          !errors.isFinite ||
-          errors < 0 ||
-          errors != errors.toInt()) {
-        throw FormatException('$method returned an invalid error count');
-      }
-      final pathValue = result['path'];
-      final countValue = result['count'];
-      if (pathValue is! String || pathValue.trim().isEmpty) {
-        throw FormatException('$method returned an invalid output path');
-      }
-      if (countValue is! num ||
-          !countValue.isFinite ||
-          countValue < 0 ||
-          countValue != countValue.toInt()) {
-        throw FormatException('$method returned an invalid row count');
-      }
-      final path = pathValue;
-      final count = countValue.toInt();
-      final file = File(path);
-      if (!await file.exists()) {
-        throw FormatException('$method did not create its output file');
-      }
-      return LibraryScanNDJSONFile(
-        file: file,
-        expectedCount: count,
-        errorCount: errors.toInt(),
-      );
-    } catch (_) {
-      // Keep partial output; native SAF scan resumes from its sidecar.
-      rethrow;
+    // Partial output is kept on failure; native SAF scan resumes from its sidecar.
+    if (isCancelled?.call() == true) {
+      throw StateError('Library scan cancelled before native scan');
     }
+    if (forceFullScan) {
+      // An explicit rescan must not resume metadata from an earlier attempt.
+      for (final file in [output, File('${output.path}.state')]) {
+        if (await file.exists()) await file.delete();
+      }
+      // Preserve covers referenced by the current DB until ingestion succeeds.
+      // Fresh paths bypass native disk and Flutter image caches together.
+      final covers = Directory('${scanDir.path}/library_covers');
+      await covers.create(recursive: true);
+      final freshCovers = await covers.createTemp('full_scan_');
+      await setLibraryCoverCacheDir(freshCovers.path);
+    }
+    if (isCancelled?.call() == true) {
+      throw StateError('Library scan cancelled before native scan');
+    }
+    final result = await _channel.invokeMethod(method, {
+      ...arguments,
+      'output_path': output.path,
+    });
+    if (result is! Map) {
+      throw FormatException('$method returned ${result.runtimeType}');
+    }
+    if (result['cancelled'] == true) {
+      throw FormatException('$method returned a cancelled partial scan');
+    }
+    final errors = result['error_count'];
+    if (errors is! num ||
+        !errors.isFinite ||
+        errors < 0 ||
+        errors != errors.toInt()) {
+      throw FormatException('$method returned an invalid error count');
+    }
+    final pathValue = result['path'];
+    final countValue = result['count'];
+    if (pathValue is! String || pathValue.trim().isEmpty) {
+      throw FormatException('$method returned an invalid output path');
+    }
+    if (countValue is! num ||
+        !countValue.isFinite ||
+        countValue < 0 ||
+        countValue != countValue.toInt()) {
+      throw FormatException('$method returned an invalid row count');
+    }
+    final path = pathValue;
+    final count = countValue.toInt();
+    final file = File(path);
+    if (!await file.exists()) {
+      throw FormatException('$method did not create its output file');
+    }
+    return LibraryScanNDJSONFile(
+      file: file,
+      expectedCount: count,
+      errorCount: errors.toInt(),
+    );
   }
 
   static Future<Map<String, dynamic>> scanSafTreeIncremental(