feat(editor): middle mouse orbit camera around terrain point

- Middle mouse drag orbits camera around the terrain point under cursor
  (or 100 units ahead if no terrain hit)
- Maintains distance from pivot while rotating yaw/pitch
- Much more intuitive for inspecting terrain features, placed objects,
  and NPC positions from different angles
- Works alongside right-drag (free look) and WASD (fly)

tools/editor/editor_app.cpp

@@ -263,7 +263,12 @@ void EditorApp::processEvents() {
         if (event.type == SDL_MOUSEMOTION && !io.WantCaptureMouse) {
             // Gizmo drag takes priority over camera
             auto& giz = viewport_.getGizmo();
-            if (giz.isDragging()) {
+            if (event.motion.state & SDL_BUTTON_MMASK) {
+                // Middle mouse = orbit around brush/terrain point
+                auto& brush = terrainEditor_.brush();
+                glm::vec3 pivot = brush.isActive() ? brush.getPosition() : camera_.getCamera().getPosition() + camera_.getCamera().getForward() * 100.0f;
+                camera_.processMiddleMouseMotion(event.motion.xrel, event.motion.yrel, pivot);
+            } else if (giz.isDragging()) {
                 auto ext = window_->getVkContext()->getSwapchainExtent();
                 giz.updateDrag(glm::vec2(static_cast<float>(event.motion.x),
                                           static_cast<float>(event.motion.y)),

tools/editor/editor_camera.cpp

@@ -71,6 +71,24 @@ void EditorCamera::processKeyEvent(const SDL_KeyboardEvent& event) {
 void EditorCamera::processMouseButton(const SDL_MouseButtonEvent& event) {
     if (event.button == SDL_BUTTON_RIGHT)
         rightMouseDown_ = (event.type == SDL_MOUSEBUTTONDOWN);
+    if (event.button == SDL_BUTTON_MIDDLE)
+        middleMouseDown_ = (event.type == SDL_MOUSEBUTTONDOWN);
+}
+
+void EditorCamera::processMiddleMouseMotion(int dx, int dy, const glm::vec3& pivotPoint) {
+    if (!middleMouseDown_) return;
+
+    constexpr float sensitivity = 0.3f;
+    yaw_ += static_cast<float>(dx) * sensitivity;
+    pitch_ -= static_cast<float>(dy) * sensitivity;
+    pitch_ = std::clamp(pitch_, -89.0f, 89.0f);
+    camera_.setRotation(yaw_, pitch_);
+
+    // Orbit: maintain distance from pivot
+    glm::vec3 toPivot = pivotPoint - camera_.getPosition();
+    float dist = glm::length(toPivot);
+    glm::vec3 newPos = pivotPoint - camera_.getForward() * dist;
+    camera_.setPosition(newPos);
 }
 
 void EditorCamera::setPosition(const glm::vec3& pos) {

tools/editor/editor_camera.hpp

@@ -16,6 +16,7 @@ public:
     void processMouseWheel(float delta, bool shiftHeld);
     void processKeyEvent(const SDL_KeyboardEvent& event);
     void processMouseButton(const SDL_MouseButtonEvent& event);
+    void processMiddleMouseMotion(int dx, int dy, const glm::vec3& pivotPoint);
 
     rendering::Camera& getCamera() { return camera_; }
     const rendering::Camera& getCamera() const { return camera_; }
@@ -33,6 +34,7 @@ private:
     bool keyW_ = false, keyA_ = false, keyS_ = false, keyD_ = false;
     bool keyQ_ = false, keyE_ = false, keyShift_ = false;
     bool rightMouseDown_ = false;
+    bool middleMouseDown_ = false;
 };
 
 } // namespace editor