feat: 转场特效引擎 — TransitionEngine + 14种转场预设 + 时长边界校验 + 降级策略

核心能力：
- TransitionEngine 抽象层：TransitionType枚举 + TransitionConfig配置 + 引擎类
- 14种转场预设：fade/dissolve/zoom/circlecrop/rectcrop + slide*4 + wipe*4
- 转场时长边界校验：0.3s~2s范围，超出自动钳制
- 降级策略：不支持的转场自动fallback到硬切(cut)，不阻断渲染
- 别名支持：slide_left/slide-left/crossfade/wipe/zoomin等多种写法

系统集成：
- UnifiedRenderService 接入 TransitionEngine，替换直接xfade调用
- EditPlanClip 领域模型新增 transition_duration 字段
- SQLAlchemy model + repository 同步新增列
- API层 Schema + Service 同步支持 transition_duration
- 向前兼容：transition_duration=0 表示使用全局默认值

测试：
- 54个新增单测：枚举/配置解析/边界校验/降级策略/滤镜链构建/集成验证
- 84个渲染测试 + 16个适配器测试全绿，无回归

packages/adapters/sqlalchemy_impl/edit_plan_clip_repository.py

@@ -54,6 +54,7 @@ class SQLAlchemyEditPlanClipRepository:
             start_time=clip.start_time,
             duration=clip.duration,
             transition_effect=clip.transition_effect,
+            transition_duration=clip.transition_duration,
             status=clip.status,
             config=clip.config,
         )
@@ -76,6 +77,7 @@ class SQLAlchemyEditPlanClipRepository:
         model.start_time = clip.start_time
         model.duration = clip.duration
         model.transition_effect = clip.transition_effect
+        model.transition_duration = clip.transition_duration
         model.status = clip.status
         model.config = clip.config
         model.updated_at = clip.updated_at
@@ -120,6 +122,7 @@ class SQLAlchemyEditPlanClipRepository:
             start_time=model.start_time or 0.0,
             duration=model.duration or 0.0,
             transition_effect=model.transition_effect or "cut",
+            transition_duration=getattr(model, "transition_duration", 0.0) or 0.0,
             status=EditPlanClipStatus(model.status) if model.status else EditPlanClipStatus.PENDING,
             config=model.config or {},
             created_at=model.created_at,

packages/adapters/sqlalchemy_impl/models.py

@@ -196,6 +196,7 @@ class EditPlanClipModel(Base):
     start_time = Column(Float, nullable=False, default=0.0)
     duration = Column(Float, nullable=False, default=0.0)
     transition_effect = Column(String(20), nullable=False, default="cut")
+    transition_duration = Column(Float, nullable=False, default=0.0)
     status = Column(String(20), nullable=False, default="pending", index=True)
     config = Column(JSON, nullable=False, default=dict)
     created_at = Column(DateTime, nullable=False, default=lambda: datetime.now(timezone.utc))