fix: 硬切场景改用concat filter替代xfade，提升多素材场景性能与稳定性

- 全硬切(no transition)场景下使用concat filter，性能远优于xfade
- 有转场效果时继续走xfade路径，保持兼容
- 修复硬切默认走xfade(fade fallback)的设计问题
- 新增单元测试覆盖concat和xfade两种路径
- 87个unified_render测试全绿，全套2147测试零回归

apps/worker/video_processing/unified_render_service.py

@@ -1363,23 +1363,39 @@ class UnifiedRenderService:
                 # 单 clip 层，直接使用预处理标签
                 layer_output_labels[layer.role] = layer_labels[0]
             else:
-                # 多 clip 层，用 TransitionEngine 构建转场链
                 out_label = f"{layer.role}_merged"
-                # 计算该层使用的转场时长（取首个非零值，否则用默认）
-                layer_dur = 0.0
-                for d in layer_transition_durations:
-                    if d > 0:
-                        layer_dur = d
-                        break
-                xfade_filter, _ = self._transition_engine.build_xfade_chain(
-                    clip_durations=layer_durations,
-                    clip_video_labels=layer_labels,
-                    transitions=layer_transitions,
-                    transition_duration=layer_dur if layer_dur > 0 else None,
-                    output_label=out_label,
+                # 判断是否全部为硬切：是则用 concat filter，否则用 xfade 转场链
+                all_cut = all(
+                    t is None or t == "" or str(t).lower() == "cut"
+                    for t in layer_transitions[1:]  # 第一个 clip 的转场忽略
                 )
-                if xfade_filter:
-                    filter_parts.append(xfade_filter)
+                if all_cut:
+                    # 全硬切：用 concat filter，性能远优于 xfade
+                    concat_inputs = "".join(f"[{label}]" for label in layer_labels)
+                    filter_parts.append(
+                        f"{concat_inputs}concat=n={len(layer_labels)}:v=1:a=0[{out_label}]"
+                    )
+                    logger.info(
+                        "[unified-render] layer=%s clips=%d using concat (all hard-cut)",
+                        layer.role,
+                        len(layer_labels),
+                    )
+                else:
+                    # 有转场效果：用 TransitionEngine 构建 xfade 链
+                    layer_dur = 0.0
+                    for d in layer_transition_durations:
+                        if d > 0:
+                            layer_dur = d
+                            break
+                    xfade_filter, _ = self._transition_engine.build_xfade_chain(
+                        clip_durations=layer_durations,
+                        clip_video_labels=layer_labels,
+                        transitions=layer_transitions,
+                        transition_duration=layer_dur if layer_dur > 0 else None,
+                        output_label=out_label,
+                    )
+                    if xfade_filter:
+                        filter_parts.append(xfade_filter)
                 layer_output_labels[layer.role] = out_label
 
         # Step 3: 合成各层

tests/unit/test_unified_render_service.py

@@ -45,6 +45,7 @@ class FakeClip:
     start_time: float = 0.0
     duration: float = 0.0
     transition_effect: str = "cut"
+    transition_duration: float = 0.0
     status: str = "ready"
     config: dict[str, Any] = field(default_factory=dict)
 
@@ -65,6 +66,7 @@ def _make_clip(
     asset_id: str = "",
     duration: float = 0.0,
     transition_effect: str = "cut",
+    transition_duration: float = 0.0,
     config: dict[str, Any] | None = None,
 ) -> FakeClip:
     return FakeClip(
@@ -74,6 +76,7 @@ def _make_clip(
         asset_id=asset_id or f"asset_{clip_id}.mp4",
         duration=duration,
         transition_effect=transition_effect,
+        transition_duration=transition_duration,
         config=config or {},
     )
 
@@ -332,7 +335,7 @@ class TestBuildFilterComplex:
         assert "[final_video]" in fc
 
     def test_single_layer_multi_clips(self):
-        """多个 main clips → xfade 串联。"""
+        """多个 main clips（默认硬切）→ concat 串联。"""
         clips = [
             _make_clip("c1", "main", order=0, duration=3.0),
             _make_clip("c2", "main", order=1, duration=3.0),
@@ -343,6 +346,35 @@ class TestBuildFilterComplex:
         }
         svc = _make_service(clips, asset_paths)
 
+        with _patch_path_exists(), patch("video_processing.unified_render_service.probe_duration", return_value=5.0):
+            resolved = svc._resolve_clips()
+            layers = svc._group_clips_into_layers(resolved)
+            fc, input_args = svc._build_filter_complex(layers)
+
+        assert input_args.count("-i") == 2
+        # 全硬切场景走 concat filter（性能远优于 xfade）
+        assert "concat=n=2:v=1:a=0" in fc
+        assert "[final_video]" in fc
+
+    def test_single_layer_multi_clips_with_transition(self):
+        """多个 main clips 带转场效果 → xfade 串联。"""
+        clips = [
+            _make_clip("c1", "main", order=0, duration=3.0),
+            _make_clip(
+                "c2",
+                "main",
+                order=1,
+                duration=3.0,
+                transition_effect="fade",
+                transition_duration=0.5,
+            ),
+        ]
+        asset_paths = {
+            "asset_c1.mp4": Path("/tmp/asset_c1.mp4"),
+            "asset_c2.mp4": Path("/tmp/asset_c2.mp4"),
+        }
+        svc = _make_service(clips, asset_paths)
+
         with _patch_path_exists(), patch("video_processing.unified_render_service.probe_duration", return_value=5.0):
             resolved = svc._resolve_clips()
             layers = svc._group_clips_into_layers(resolved)
@@ -380,7 +412,7 @@ class TestBuildFilterComplex:
         """
         clips = [
             _make_clip("c1", "main", order=0, duration=3.0),
-            _make_clip("c2", "main", order=1, duration=5.0),
+            _make_clip("c2", "main", order=1, duration=5.0, transition_effect="fade", transition_duration=0.5),
         ]
         asset_paths = {
             "asset_c1.mp4": Path("/tmp/asset_c1.mp4"),