perf(analysis): serialize and cancel owned FFmpeg sessions

test/audio_analysis_jobs_test.dart

@@ -0,0 +1,74 @@
+import 'dart:async';
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:spotiflac_android/services/audio_analysis_jobs.dart';
+
+void main() {
+  test(
+    'replacement waits for cancelled session and skips stale queued work',
+    () async {
+      final completions = <Completer<String>>[];
+      final started = <String>[];
+      final cancelled = <int>[];
+      final jobs = AudioAnalysisJobs<String>(
+        start: (args) async {
+          started.add(args.single);
+          final done = Completer<String>();
+          completions.add(done);
+          return (id: completions.length, completed: done.future);
+        },
+        cancel: (id) async => cancelled.add(id),
+      );
+      final first = jobs.run(['old']);
+      final firstCheck = expectLater(
+        first,
+        throwsA(isA<AudioAnalysisCancelled>()),
+      );
+      await Future<void>.delayed(Duration.zero);
+      final stale = jobs.run(['stale']);
+      final staleCheck = expectLater(
+        stale,
+        throwsA(isA<AudioAnalysisCancelled>()),
+      );
+      jobs.invalidate();
+      final latest = jobs.run(['latest']);
+      await Future<void>.delayed(Duration.zero);
+      expect(cancelled, [1]);
+      expect(started, ['old']);
+      completions.first.complete('discarded');
+      await firstCheck;
+      await staleCheck;
+      await Future<void>.delayed(Duration.zero);
+      expect(started, ['old', 'latest']);
+      completions.last.complete('result');
+      expect(await latest, 'result');
+      jobs.dispose();
+    },
+  );
+
+  test(
+    'dispose during session creation cancels its eventual session ID',
+    () async {
+      final creation = Completer<({int id, Future<String> completed})>();
+      final completion = Completer<String>();
+      final cancelled = <int>[];
+      final jobs = AudioAnalysisJobs<String>(
+        start: (_) => creation.future,
+        cancel: (id) async => cancelled.add(id),
+      );
+      final result = jobs.run(['analysis']);
+      final check = expectLater(result, throwsA(isA<AudioAnalysisCancelled>()));
+      await Future<void>.delayed(Duration.zero);
+      jobs.dispose();
+      creation.complete((id: 42, completed: completion.future));
+      await Future<void>.delayed(Duration.zero);
+      expect(cancelled, [42]);
+      completion.complete('discarded');
+      await check;
+      await expectLater(
+        jobs.run(['later']),
+        throwsA(isA<AudioAnalysisCancelled>()),
+      );
+    },
+  );
+}