ifcviewer: multi-selection + box-select with active highlight

SelectionState (new) owns the multi-set, the "active" id (last single-
clicked), and a per-object_id flags SSBO bound at binding=3.  Main
shader reads sel_flags[v_object_id] for the in-set tint and a separate
u_active_id uniform for a stronger tint on the active.

Click semantics: plain replaces, Shift/Ctrl toggles.  LMB-drag past 5px
boxes the rect through a pick-pass readback — plain replaces, Shift
adds, Ctrl removes; box-select preserves the active.  Drag promotes
regardless of start point so a press on geometry doesn't disqualify it.

Sidecar fast-path bulk-loads instances, so noteObjectId is also called
from the apply path — without it the flags buffer was sized to 1 slot
while object_ids were in the 100k+ range and the in-set bit was lost.

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

src/ifcviewer/OverlayRenderer.h

@@ -20,6 +20,7 @@
 #ifndef IFCVIEWER_OVERLAYRENDERER_H
 #define IFCVIEWER_OVERLAYRENDERER_H
 
+#include <QRect>
 #include <QString>
 #include <QtOpenGL/QOpenGLFunctions_4_5_Core>
 
@@ -86,6 +87,12 @@ public:
     // render().  Empty hides the HUD.
     void setHudText(const QString& text);
 
+    // Box-select rectangle (in logical pixel coords, top-left origin).
+    // Drawn as a translucent fill + 1-px outline using the same
+    // screen-space rect program that draws label/HUD backgrounds.
+    // Empty rect hides it.
+    void setSelectionRect(const QRect& rect_logical);
+
     // Render every overlay primitive in order: GL highlight triangles
     // (using `view_proj`, column-major float[16]), then HUD text via
     // QPainter on a QOpenGLPaintDevice sized to (pixel_w × pixel_h)
@@ -182,6 +189,9 @@ private:
 
     std::vector<Label> labels_;
     QString            hud_text_;
+
+    // Box-select rectangle in logical pixels.  Null/empty = hidden.
+    QRect              selection_rect_;
 };
 
 #endif // IFCVIEWER_OVERLAYRENDERER_H