ifcviewer-full: volume tool with HUD + per-object labels

Mirrors the Area tool's display: HUD shows total volume + object count,
each selected object gets a label at its world-AABB centroid showing
its individual volume.  Gated behind ToolMode::Volume (Ctrl+Shift+V) so
it stays out of the way until invoked.

Volume is a passive tool — selection works as in None (multi-select,
modifier toggle, box-select all keep working).  Area / Length still
intercept clicks through surfacePickedInTool.

Adds volumesPerObject() reusing the same mesh-cached readback path as
volumeOfObjects, so the per-object split costs no extra GL readbacks.
computeObjectAabb is promoted to public for the centroid lookup.

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

src/ifcviewer/ViewportWindow.cpp

@@ -3596,8 +3596,11 @@ void ViewportWindow::handleMousePress(QMouseEvent* e) {
     // drag that happens to start on an object still box-selects, which
     // matches user intuition: the start point shouldn't disqualify the
     // gesture.  Tool-mode LMB defers pick handling to surfacePickedInTool
-    // on release.
-    if (e->button() == Qt::LeftButton && tool_mode_ == ToolMode::None) {
+    // on release for tools that consume clicks (Area / Length); Volume is
+    // passive and routes input through normal selection.
+    const bool tool_consumes_clicks =
+        (tool_mode_ == ToolMode::Area || tool_mode_ == ToolMode::Length);
+    if (e->button() == Qt::LeftButton && !tool_consumes_clicks) {
         press_pick_id_ = pickObjectAt(e->pos().x(), e->pos().y());
         box_select_start_pos_   = e->pos();
         box_select_current_pos_ = e->pos();
@@ -3637,8 +3640,11 @@ void ViewportWindow::handleMouseRelease(QMouseEvent* e) {
                 selection_.setSelection(picks, selection_.activeObjectId());
             }
         } else if (!was_drag) {
-            // Click — apply press-time pick + modifiers.
-            if (tool_mode_ != ToolMode::None) {
+            // Click — apply press-time pick + modifiers.  Area / Length
+            // intercept clicks; None and Volume drive normal selection.
+            const bool tool_consumes_clicks =
+                (tool_mode_ == ToolMode::Area || tool_mode_ == ToolMode::Length);
+            if (tool_consumes_clicks) {
                 emit surfacePickedInTool(e->pos().x(), e->pos().y(),
                                          int(e->modifiers()));
             } else {
@@ -4225,6 +4231,10 @@ void ViewportWindow::toggleLengthTool() {
     setToolMode(tool_mode_ == ToolMode::Length ? ToolMode::None : ToolMode::Length);
 }
 
+void ViewportWindow::toggleVolumeTool() {
+    setToolMode(tool_mode_ == ToolMode::Volume ? ToolMode::None : ToolMode::Volume);
+}
+
 void ViewportWindow::setHighlightTriangles(const std::vector<float>& world_xyz,
                                             float r, float g, float b, float a) {
     if (!gl_initialized_) return;

src/ifcviewer/ViewportWindow.h

@@ -255,11 +255,15 @@ public:
     // object selection); the app interprets them per-tool.  Esc exits the
     // active tool.  Backspace/Delete in length mode emits
     // toolBackspacePressed for "remove last point" semantics.
-    enum class ToolMode { None, Area, Length };
+    // Area / Length consume LMB clicks via surfacePickedInTool; Volume
+    // is passive — selection behaves as in None and the host just gates
+    // its volume HUD/labels on this mode.
+    enum class ToolMode { None, Area, Length, Volume };
     Q_ENUM(ToolMode)
 
     void toggleAreaTool();
     void toggleLengthTool();
+    void toggleVolumeTool();
     void setToolMode(ToolMode mode);
     ToolMode toolMode() const { return tool_mode_; }
 
@@ -379,6 +383,10 @@ public:
     // Frame the union of all finalized models.  No-op if the scene is empty.
     void viewAll();
 
+    // World-space AABB query.  Returns false when the object has no live
+    // instance or no mesh AABB yet (caller should treat as "unknown").
+    bool computeObjectAabb(uint32_t object_id, QVector3D& mn, QVector3D& mx) const;
+
     struct CameraState {
         QVector3D target;
         float distance;
@@ -466,10 +474,8 @@ private:
     void updateSectionDrag(int x, int y);
     void updateCamera();
 
-    // Geometry queries used by focusOnSelectedObject() / viewAll().  Both
-    // return false when nothing matched (caller should leave the camera
-    // alone).  Bounds are world-space AABBs.
-    bool computeObjectAabb(uint32_t object_id, QVector3D& mn, QVector3D& mx) const;
+    // Scene-wide AABB used by viewAll().  Returns false when the scene
+    // has no finalized geometry (caller should leave the camera alone).
     bool computeSceneAabb(QVector3D& mn, QVector3D& mx) const;
     // Re-aim the orbit camera so the bounding sphere of [mn, mx] just fits
     // vertically and horizontally within the current FOV, with `padding`