feat(app): bounded-parallel prepare (default 3) replaces serial queue

Co-Authored-By: Claude Code <noreply@anthropic.com>

apps/mobile/test/features/uploads/preparer_test.dart

@@ -47,11 +47,15 @@ void main() {
     return f;
   }
 
-  UploadPreparer preparer({int maxBytes = UploadPreparer.kMaxUploadBytes}) =>
+  UploadPreparer preparer({
+    int maxBytes = UploadPreparer.kMaxUploadBytes,
+    int maxConcurrentPrepares = kMaxConcurrentPrepares,
+  }) =>
       UploadPreparer(
         store,
         baseDirProvider: () async => managedBase,
         maxBytes: maxBytes,
+        maxConcurrentPrepares: maxConcurrentPrepares,
       );
 
   group('prepare（快乐路径）', () {
@@ -216,9 +220,66 @@ void main() {
     });
   });
 
-  group('串行执行（连拍防 IO 风暴）', () {
-    test('并发 prepare 按提交顺序串行完成，各自独立成行', () async {
-      final p = preparer();
+  group('有界并发执行（连拍防 IO 风暴的上限钳位）', () {
+    /// 构造带并发探针的准备器：baseDirProvider 在 _prepare 内被 await，
+    /// 其重叠度是「在途 prepare 数」的可观测下界（且受信号量上限约束）。
+    (UploadPreparer, int Function()) probePreparer(int limit) {
+      var active = 0;
+      var peak = 0;
+      final p = UploadPreparer(
+        store,
+        baseDirProvider: () async {
+          active++;
+          if (active > peak) peak = active;
+          await Future<void>.delayed(const Duration(milliseconds: 30));
+          active--;
+          return managedBase;
+        },
+        maxConcurrentPrepares: limit,
+      );
+      return (p, () => peak);
+    }
+
+    test('并发 prepare 峰值 = maxConcurrentPrepares（≤上限且真并行），9 任务全部 queued', () async {
+      final (p, peakOf) = probePreparer(3);
+      final srcs = [
+        for (var i = 0; i < 9; i++)
+          sourceFile('par-$i.jpg', _jpgBytes + [i]),
+      ];
+      final tasks = await Future.wait([
+        for (final src in srcs)
+          p.prepare(accountId: 'alice', albumId: 'alb-1', sourcePath: src.path),
+      ]);
+      // 全部入队、ID/幂等键互不相同、副本都在。
+      expect(tasks.map((t) => t.state), everyElement(UploadTaskState.queued));
+      expect(tasks.map((t) => t.id).toSet().length, 9);
+      expect(tasks.map((t) => t.clientTaskId).toSet().length, 9);
+      for (final t in tasks) {
+        expect(File(t.sourceFilePath).existsSync(), isTrue);
+      }
+      // 峰值恰为 3：≤3 证明上限钳位（防连拍 IO 风暴初衷保留）；
+      // 达到 3 证明未退化为串行。
+      expect(peakOf(), 3);
+    });
+
+    test('maxConcurrentPrepares=1 等价旧串行行为：峰值 1、任务仍全部成功', () async {
+      final (p, peakOf) = probePreparer(1);
+      final srcs = [
+        for (var i = 0; i < 4; i++)
+          sourceFile('ser-$i.jpg', _jpgBytes + [i]),
+      ];
+      final tasks = await Future.wait([
+        for (final src in srcs)
+          p.prepare(accountId: 'alice', albumId: 'alb-1', sourcePath: src.path),
+      ]);
+      expect(tasks.map((t) => t.state), everyElement(UploadTaskState.queued));
+      expect(peakOf(), 1, reason: '并发 1 时任意时刻在途 prepare 至多 1（串行）');
+    });
+  });
+
+  group('串行回归（maxConcurrentPrepares=1 等价旧全局串行队列）', () {
+    test('并发=1：prepare 按提交顺序串行完成，各自独立成行', () async {
+      final p = preparer(maxConcurrentPrepares: 1);
       final srcs = [
         for (var i = 0; i < 3; i++)
           sourceFile('burst-$i.jpg', _jpgBytes + [i]),
@@ -234,7 +295,7 @@ void main() {
       for (final t in tasks) {
         expect(File(t.sourceFilePath).existsSync(), isTrue);
       }
-      // createdAt 严格递增（串行执行的间接证据：无交叉写库）。
+      // createdAt 非降（并发=1 串行的间接证据：无交叉写库）。
       final stamps = tasks.map((t) => t.createdAt).toList();
       for (var i = 1; i < stamps.length; i++) {
         expect(!stamps[i].isBefore(stamps[i - 1]), isTrue);