ifcviewer: move section-plane mutators into ViewportCore (#84-y)

addSectionPlaneAtSurface (camera-facing auto-flip + kMaxSectionPlanes
cap check), removeSectionPlane, and clearSectionPlanes all move to
ViewportCore. ViewportWindow keeps tiny forwarders so the section
tool's input handlers (still VW) call through without seeing the move.

Each method now calls host_->requestFrame() in place of the
isExposed() + requestUpdate() gate, which means the section tool path
becomes the next piece that could exercise the WebViewportHost: a
click-to-add over WebGPU will work as soon as the host is wired,
without further core-side changes.

src/ifcviewer/ViewportCore.cpp

@@ -5268,3 +5268,52 @@ void ViewportCore::render() {
         }
     }
 }
+
+// ===========================================================================
+// Section planes (#84-y)
+// ===========================================================================
+
+bool ViewportCore::addSectionPlaneAtSurface(const Eigen::Vector3f& point,
+                                            const Eigen::Vector3f& normal,
+                                            float visual_radius) {
+    if (int(section_planes_.size()) >= kMaxSectionPlanes) {
+        Log::warn() << "[wgpu section] cap reached (" << kMaxSectionPlanes
+                    << " planes)";
+        return false;
+    }
+    Eigen::Vector3f n = normal;
+    if (n.squaredNorm() < 1e-8f) return false;
+    n.normalize();
+    // Auto-flip the normal so the camera-facing half gets cut away.
+    const Eigen::Vector3f eye = orbitEye(camera_target_, camera_distance_,
+                                         camera_yaw_deg_, camera_pitch_deg_);
+    const Eigen::Vector3f eye_dir = eye - point;
+    if (n.dot(eye_dir) < 0.0f) n = -n;
+
+    SectionPlane p;
+    p.n             = n;
+    p.origin        = point;
+    p.d             = -n.dot(point);
+    p.visual_radius = (visual_radius > 0.0f) ? visual_radius : 1.0f;
+    section_planes_.push_back(p);
+    Log::info()
+        << "[wgpu section] added plane #" << section_planes_.size() - 1
+        << " origin=(" << point.x() << "," << point.y() << "," << point.z() << ")"
+        << " normal=(" << n.x() << "," << n.y() << "," << n.z() << ")";
+    host_->requestFrame();
+    return true;
+}
+
+void ViewportCore::removeSectionPlane(int index) {
+    if (index < 0 || index >= int(section_planes_.size())) return;
+    section_planes_.erase(section_planes_.begin() + index);
+    Log::info() << "[wgpu section] removed plane " << index;
+    host_->requestFrame();
+}
+
+void ViewportCore::clearSectionPlanes() {
+    if (section_planes_.empty()) return;
+    section_planes_.clear();
+    Log::info() << "[wgpu section] cleared all planes";
+    host_->requestFrame();
+}