test(compare-alert): 补 review 缺口——continue 分支回归 + <1s 边界

- read_stuck_points 最忙平台末帧损坏 → last fallthrough 到干净平台
  (重构最险分支的回归测试,原只在一次性验证脚本里)
- _fmt_last round 边界:500→<1s、999→停留1s
- continue 注释点明与旧版 _platform_stuck→None 等价

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/services/trace_stuck.py

@@ -136,7 +136,9 @@ def read_stuck_points(trace_dir: Path, *, threshold: int, max_tail: int) -> Stuc
             any_frames = True
             seg = _last_segment(step_files, max_tail)  # 每平台只扫一次末段
             if seg is None:
-                continue  # 末帧抠不出:不判卡死、也不当末帧候选
+                # 末帧抠不出:整段跳过(不判卡死、也不当末帧候选)——与旧版 _platform_stuck→None
+                # + 独立 _read_head(末帧)→ps None 两处一并跳过等价(旧版两者都 key off 末帧)
+                continue
             ps, pg, count, dwell_ms = seg
             if count >= threshold:
                 points.append(StuckPoint(pdir.name, ps, count, dwell_ms, detected_page=pg))

tests/test_compare_alert_stuck_worker.py

@@ -167,6 +167,13 @@ def test_fmt_last_sub_second():
     sp = StuckPoint("meituan", "checkout", frames=480,
                     detected_page="checkout_page", dwell_ms=300)
     assert _fmt_last(sp) == "美团·结算·checkout_page 停留<1s"
+    # <1s 阈值边界:round(500/1000)=0 → <1s;round(999/1000)=1 → 停留1s
+    sp500 = StuckPoint("meituan", "checkout", frames=480,
+                       detected_page="checkout_page", dwell_ms=500)
+    assert _fmt_last(sp500) == "美团·结算·checkout_page 停留<1s"
+    sp999 = StuckPoint("meituan", "checkout", frames=480,
+                       detected_page="checkout_page", dwell_ms=999)
+    assert _fmt_last(sp999) == "美团·结算·checkout_page 停留1s"
 
 
 def test_fmt_last_without_dwell():

tests/test_trace_stuck.py

@@ -222,3 +222,21 @@ def test_read_stuck_points_last_has_dwell(tmp_path):
     assert res.last.pipeline_step == "checkout"
     assert res.last.frames == 5           # 总帧数
     assert res.last.dwell_ms == 4000      # 末段 :00→:04 = 4s
+
+
+def test_read_stuck_points_last_falls_through_when_busiest_last_frame_corrupt(tmp_path):
+    # 最忙平台末帧损坏(抠不出环节)→ _last_segment=None → 整段跳过(重构 continue 分支)
+    # → last 落到次忙的干净平台。锁定 read_stuck_points 重构的最险等价分支。
+    m = tmp_path / "meituan"
+    m.mkdir()
+    for i in range(7):
+        _frame(m, i, "add_one_dish", "menu")
+    (m / "step_007.json").write_bytes(b'{"pipeline_step": "add\xff')  # 末帧截断 UTF-8
+    e = tmp_path / "eleme"
+    for i in range(3):
+        _frame(e, i, "enter_store", "store")
+    res = read_stuck_points(tmp_path, threshold=15, max_tail=40)
+    assert res.points == []                    # 谁都没卡死
+    assert res.last is not None
+    assert res.last.platform == "eleme"        # 最忙的 meituan(8帧)末帧损坏被跳过,last 落到 eleme
+    assert res.last.pipeline_step == "enter_store"