ifcviewer: move buildModelBindGroup + captureNextFrameToPng setter into ViewportCore (#84-v)

Two small helpers + the screenshot-quit flag flip across; the render-
side capture encode + readback + PNG save itself stays in VW (those
need a stbi-style PNG writer to replace QImage::save before they can
move, and that's its own commit).

VW keeps tiny forwarders so bonsai's SceneLoader + the CLI screenshot
path don't see the move. The streaming sync-fallback gate already
reads the core-side pending_screenshot_path_, so capture timing is
unchanged.

src/ifcviewer/ViewportWindow.h

@@ -937,7 +937,7 @@ private:
     // observes the non-empty value to switch into the sync chunk-load
     // fallback so the first-frame capture isn't an empty buffer.
     std::string& pending_screenshot_path_;
-    bool    pending_screenshot_quit_ = false;
+    bool&        pending_screenshot_quit_;
 
     // Mouse navigation state. LMB drag orbits, MMB drag pans, wheel zooms.
     // LMB-click-without-drag picks the object under the cursor. No