ifcviewer: viewport overlay subsystem (highlight tris + HUD text)

New OverlayRenderer module owns every client-supplied overlay primitive
drawn after the main pass: tinted, depth-aware highlight triangles via
its own GL shader, and top-left HUD text via QPainter on a
QOpenGLPaintDevice.  Public surface on ViewportWindow is just two
forwarders (setHighlightTriangles, setHudText).

ViewportWindow's MeshLocalPick now exposes the instance's composed
transform so consumers can map mesh-local geometry back to world
space without re-querying.  AreaMeasurement uses both: its selection
key is now (object_id, tri) so per-instance highlighting works for
two distinct walls sharing a mesh, and on every pick it rebuilds the
world-space tri list and the HUD readout.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/ifcviewer-full/MainWindow.cpp

@@ -210,11 +210,20 @@ void MainWindow::setupUi() {
             [this](int x, int y, int modifiers) {
         const bool alt = (modifiers & Qt::AltModifier) != 0;
         area_measurement_.onPick(*viewport_, x, y, alt);
+        viewport_->setHudText(QString("Area: %1 m²  (%2 tris)")
+            .arg(area_measurement_.totalArea(), 0, 'f', 4)
+            .arg(area_measurement_.triangleCount()));
     });
     connect(viewport_, &ViewportWindow::areaToolToggled, this,
             [this](bool active) {
-        area_measurement_.clear();
-        qInfo("Area tool %s", active ? "on (LMB to add patch, Alt+LMB single tri, click again to remove, Esc exits)" : "off");
+        area_measurement_.clear(*viewport_);
+        if (active) {
+            viewport_->setHudText("Area: 0.0000 m²  (0 tris)");
+            status_label_->setText("Area tool: LMB add, Alt+LMB single tri, click again to remove, Esc exits");
+        } else {
+            viewport_->setHudText(QString());
+            status_label_->setText("Ready");
+        }
     });
 
     auto* tree_dock = new QDockWidget("Elements", this);