test(client): 用例名统一为 should_<期望>_when_<条件>（155/155）

为什么：红线第 8 条门禁立即生效，要求 Flutter 测试命名合规；此前 155 条用例名是英文自然语言
或中文描述，合规数 0/155，属存量欠账，按"先补齐再上新功能"在本批处理。

- 25 个测试文件、155 条用例改名，按「文件 + 旧名」精确定位，每条断言命中恰好 1 次才替换
- 只改名字字面量：改动行 155 删 + 155 增，无一行不含字符串字面量（断言与用例体零改动）
- 门禁：dart analyze lib test = No issues found；flutter test = 155 全过

test/playback/playback_preload_test.dart

@@ -17,7 +17,7 @@ void main() {
     ),
   );
 
-  test('warm playback starts before old native disposal finishes', () async {
+  test('should_start_warm_playback_when_old_native_disposal_is_still_running', () async {
     final release = Completer<void>();
     final factory = _Factory(disposeGates: {'ep-0': release});
     final session = PlaybackSession(
@@ -41,7 +41,7 @@ void main() {
     await session.close();
   });
 
-  test('failed native release blocks another route from allocating', () async {
+  test('should_block_another_route_from_allocating_when_native_release_fails', () async {
     final factory = _Factory(disposeFailures: {'ep-0'});
     final coordinator = PlaybackCoordinator();
     PlaybackSession createSession() => PlaybackSession(
@@ -68,7 +68,7 @@ void main() {
   });
 
   test(
-    'next is prepared silently and promoted without another initialize',
+    'should_prepare_next_silently_and_promote_it_when_swiped',
     () async {
       final factory = _Factory();
       final prepared = <String>[];
@@ -103,7 +103,7 @@ void main() {
     },
   );
 
-  test('in-flight preload is reused when swiped before ready', () async {
+  test('should_reuse_in_flight_preload_when_swiped_before_ready', () async {
     final gate = Completer<void>();
     final factory = _Factory(gates: {'ep-1': gate});
     final session = PlaybackSession(
@@ -127,7 +127,7 @@ void main() {
     await session.close();
   });
 
-  test('rapid reverse and skipped selection discard stale warm work', () async {
+  test('should_discard_stale_warm_work_when_reversing_rapidly_or_skipping_selection', () async {
     final gate = Completer<void>();
     final factory = _Factory(gates: {'ep-1': gate});
     final session = PlaybackSession(
@@ -153,7 +153,7 @@ void main() {
   });
 
   test(
-    'category adjacency and locked next never preload the wrong episode',
+    'should_never_preload_wrong_episode_when_next_is_locked_or_category_based',
     () async {
       final factory = _Factory();
       final session = PlaybackSession(
@@ -172,7 +172,7 @@ void main() {
     },
   );
 
-  test('preload failure is isolated and selecting it retries once', () async {
+  test('should_isolate_preload_failure_and_retry_once_when_selected', () async {
     final factory = _Factory(failOnce: {'ep-1'});
     final session = PlaybackSession(
       episodes: episodes,
@@ -190,7 +190,7 @@ void main() {
     await session.close();
   });
 
-  test('completion in background never starts either player', () async {
+  test('should_start_no_player_when_completion_happens_in_background', () async {
     final gate = Completer<void>();
     final factory = _Factory(gates: {'ep-1': gate});
     final session = PlaybackSession(
@@ -215,7 +215,7 @@ void main() {
     await session.close();
   });
 
-  test('closing during preloading disposes pending resources', () async {
+  test('should_dispose_pending_resources_when_closing_during_preload', () async {
     final gate = Completer<void>();
     final factory = _Factory(gates: {'ep-1': gate});
     final session = PlaybackSession(
@@ -233,7 +233,7 @@ void main() {
   });
 
   test(
-    'home and series share a two-player budget across route handoff',
+    'should_share_two_player_budget_when_handing_off_between_home_and_series',
     () async {
       final factory = _Factory();
       final coordinator = PlaybackCoordinator();