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

App/editor/InspectorPanel.cpp

@@ -116,6 +116,11 @@ void InspectorPanel::Draw(const EditorContext& ctx, entt::entity selected)
         {
             reg.emplace<Engine::ECS::RigidBodyComponent>(selected);
         }
+        if (!reg.all_of<Engine::ECS::JointComponent>(selected) &&
+            ImGui::MenuItem("Joint"))
+        {
+            reg.emplace<Engine::ECS::JointComponent>(selected);
+        }
         ImGui::EndPopup();
     }
     ImGui::Separator();
@@ -334,6 +339,58 @@ void InspectorPanel::Draw(const EditorContext& ctx, entt::entity selected)
         }
     }
 
+    // ---- Joint（约束：两端 body 就绪后由 PhysicsWorld 对账创建）----
+    if (auto* jc = reg.try_get<Engine::ECS::JointComponent>(selected))
+    {
+        if (ImGui::CollapsingHeader("Joint", ImGuiTreeNodeFlags_DefaultOpen))
+        {
+            int type = static_cast<int>(jc->type);
+            if (ImGui::Combo("type", &type, "Fixed\0Hinge\0Ball\0"))
+            {
+                jc->type = static_cast<Engine::ECS::JointComponent::Type>(type);
+            }
+
+            // target 实体下拉：实体名列表，选中态按 UUID 匹配（变更写回）
+            std::string items;
+            std::vector<Engine::UUID> uuids;
+            int current = -1;
+            reg.view<Engine::ECS::IdComponent, Engine::ECS::NameComponent>().each(
+                [&](entt::entity e, const Engine::ECS::IdComponent& id,
+                    const Engine::ECS::NameComponent& n)
+                {
+                    if (e == selected)
+                    {
+                        return;   // 自连无意义
+                    }
+                    if (id.uuid == jc->target)
+                    {
+                        current = static_cast<int>(uuids.size());
+                    }
+                    uuids.push_back(id.uuid);
+                    items.append(n.name);
+                    items.push_back('\0');
+                });
+            items.push_back('\0');   // ImGui Combo 双 \0 结尾
+            if (ImGui::Combo("target", &current, items.c_str()) && current >= 0)
+            {
+                jc->target = uuids[static_cast<size_t>(current)];
+            }
+
+            ImGui::DragFloat3("anchor A", &jc->anchorA.x, 0.05f);
+            ImGui::DragFloat3("anchor B", &jc->anchorB.x, 0.05f);
+            if (jc->type == Engine::ECS::JointComponent::Type::Hinge)
+            {
+                ImGui::DragFloat3("hinge axis", &jc->hingeAxis.x, 0.05f);
+                ImGui::Checkbox("enable limit", &jc->enableLimit);
+                if (jc->enableLimit)
+                {
+                    ImGui::DragFloat("min deg", &jc->limitMin, 1.0f, -180.0f, 0.0f);
+                    ImGui::DragFloat("max deg", &jc->limitMax, 1.0f, 0.0f, 180.0f);
+                }
+            }
+        }
+    }
+
     // ---- Animation ----
     if (auto* anim = reg.try_get<Engine::ECS::AnimationComponent>(selected))
     {