physics: joint constraints (fixed/hinge/ball)

- JointComponent: type, target UUID, per-body local anchors, hinge axis
  + angle limits (data only, zero Jolt deps)
- PhysicsWorld::SyncConstraints: create/remove/rebuild on snapshot
  change; body rebuild contagion via invalidated-bits list; target
  resolved lazily by UUID each sync (no two-pass load needed)
- Jolt FixedConstraint/HingeConstraint/PointConstraint in
  LocalToBodyCOM space (anchors minus shape COM); fixed frames lock
  creation-time relative pose; hinge axis aligned across bodies
- Inspector: Joint section with target entity name dropdown
- Serialization: Joint map with UUID target, as<T> fallbacks

Engine/src/Serialization/SceneSerializer.cpp

@@ -307,6 +307,27 @@ namespace Engine::Serialization {
                 out << YAML::EndMap;
             }
 
+            if (const auto* jc = reg.try_get<ECS::JointComponent>(e)) {
+                out << YAML::Key << "Joint" << YAML::Value
+                    << YAML::BeginMap;
+                out << YAML::Key << "Type" << YAML::Value
+                    << static_cast<int>(jc->type);
+                // 实体引用存 UUID（跨存档稳定；target=0 未设置）
+                out << YAML::Key << "Target" << YAML::Value
+                    << jc->target.Value();
+                out << YAML::Key << "AnchorA" << YAML::Value;
+                EmitVec3(out, jc->anchorA);
+                out << YAML::Key << "AnchorB" << YAML::Value;
+                EmitVec3(out, jc->anchorB);
+                out << YAML::Key << "HingeAxis" << YAML::Value;
+                EmitVec3(out, jc->hingeAxis);
+                out << YAML::Key << "EnableLimit" << YAML::Value
+                    << jc->enableLimit;
+                out << YAML::Key << "LimitMin" << YAML::Value << jc->limitMin;
+                out << YAML::Key << "LimitMax" << YAML::Value << jc->limitMax;
+                out << YAML::EndMap;
+            }
+
             out << YAML::EndMap;   // Components
             out << YAML::EndMap;   // Entity
         }
@@ -562,6 +583,24 @@ namespace Engine::Serialization {
                 rb.isSensor = n["IsSensor"].as<bool>(rb.isSensor);
                 // bodyId/bodyValid/velocity 保持默认（对账重建）
             }
+
+            // Joint：target 只存 UUID，加载后由 PhysicsWorld 惰性反查
+            //（两遍解析不需要——UUID 再生/加载顺序均由逐帧解析消化）
+            if (const YAML::Node n = comps["Joint"]) {
+                auto& jc = handle.AddComponent<ECS::JointComponent>();
+                if (const int type = n["Type"].as<int>(0);
+                    type >= 0 && type <= 2) {
+                    jc.type =
+                        static_cast<ECS::JointComponent::Type>(type);
+                }
+                jc.target = UUID{n["Target"].as<u64>(0)};
+                jc.anchorA = ReadVec3(n["AnchorA"], jc.anchorA);
+                jc.anchorB = ReadVec3(n["AnchorB"], jc.anchorB);
+                jc.hingeAxis = ReadVec3(n["HingeAxis"], jc.hingeAxis);
+                jc.enableLimit = n["EnableLimit"].as<bool>(jc.enableLimit);
+                jc.limitMin = n["LimitMin"].as<f32>(jc.limitMin);
+                jc.limitMax = n["LimitMax"].as<f32>(jc.limitMax);
+            }
         }
 
         // 加载在 OnImGuiRender 段，错过本帧 UpdateTransforms：