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