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/ViewportCore.cpp

@@ -4582,3 +4582,24 @@ void ViewportCore::configureSurface(int width_px, int height_px) {
         edge_bind_group_ = nullptr;
     }
 }
+
+// ===========================================================================
+// Small cross-chunk + capture helpers (#84-v)
+// ===========================================================================
+
+void ViewportCore::buildModelBindGroup(ModelGpuData& m) {
+    if (!m.mesh_storage || !m.instance_storage) {
+        // Empty model — no chunks, no bind groups; the draw loop will skip.
+        return;
+    }
+    for (std::size_t ci = 0; ci < m.chunks.size(); ++ci) {
+        buildChunkBindGroup(m, ci);
+    }
+}
+
+void ViewportCore::captureNextFrameToPng(const std::string& path,
+                                         bool quit_after) {
+    pending_screenshot_path_ = path;
+    pending_screenshot_quit_ = quit_after;
+    host_->requestFrame();
+}

src/ifcviewer/ViewportCore.h

@@ -323,6 +323,21 @@ public:
     void uploadInstanceChunk(const InstanceChunk& chunk);
     void finalizeModel(std::uint32_t model_id);
 
+    // ---- Cross-chunk + screenshot capture (#84-v) -------------------------
+    //
+    // Rebuild every chunk's bind group for the supplied model. No-op
+    // when the model has no GPU storage yet (empty load — the chunk
+    // draw loop skips it anyway).
+    void buildModelBindGroup(ModelGpuData& m);
+
+    // Arm a one-shot screenshot capture. The next render() encodes a
+    // surface-to-buffer copy alongside the main pass, maps it back to
+    // RGBA8, and saves a PNG at `path`. `quit_after` requests host
+    // shutdown once the capture writes — the host's quit() decides
+    // when (synchronously or queued).
+    void captureNextFrameToPng(const std::string& path, bool quit_after);
+    bool pending_screenshot_quit_ = false;
+
     // ---- Surface configuration (#84-u) ------------------------------------
     //
     // Configure the swapchain at the given physical size. Picks a present

src/ifcviewer/ViewportWindow.cpp

@@ -247,6 +247,7 @@ ViewportWindow::ViewportWindow(QWindow* parent)
       streaming_blocked_oom_this_frame_(core_.streaming_blocked_oom_this_frame_),
       streaming_debug_                 (core_.streaming_debug_),
       pending_screenshot_path_(core_.pending_screenshot_path_),
+      pending_screenshot_quit_(core_.pending_screenshot_quit_),
       lod1_dbg_count_         (core_.lod1_dbg_count_),
       lod0_dbg_eligible_count_(core_.lod0_dbg_eligible_count_),
       lod0_dbg_no_lod1_count_ (core_.lod0_dbg_no_lod1_count_),
@@ -2431,15 +2432,7 @@ void ViewportWindow::ensureSelectionFlagsBuffer() { core_.ensureSelectionFlagsBu
 // uploadSelectionFlagsIfDirty moved to ViewportCore (#84-k).
 void ViewportWindow::uploadSelectionFlagsIfDirty() { core_.uploadSelectionFlagsIfDirty(); }
 
-void ViewportWindow::buildModelBindGroup(ModelGpuData& m) {
-    if (!m.mesh_storage || !m.instance_storage) {
-        // Empty model — no chunks, no bind groups; the draw loop will skip.
-        return;
-    }
-    for (size_t ci = 0; ci < m.chunks.size(); ++ci) {
-        core_.buildChunkBindGroup(m, ci);
-    }
-}
+void ViewportWindow::buildModelBindGroup(ModelGpuData& m) { core_.buildModelBindGroup(m); }
 
 // buildChunkBindGroup moved to ViewportCore (#84-n).
 
@@ -2685,11 +2678,7 @@ void ViewportWindow::applyNavPreset(const char* name) {
 #include <QImage>
 #include <QCoreApplication>
 
-void ViewportWindow::captureNextFrameToPng(const std::string& path, bool quit_after) {
-    pending_screenshot_path_ = path;
-    pending_screenshot_quit_ = quit_after;
-    if (isExposed()) requestUpdate();
-}
+void ViewportWindow::captureNextFrameToPng(const std::string& path, bool quit_after) { core_.captureNextFrameToPng(path, quit_after); }
 
 // -----------------------------------------------------------------------------
 // Mouse navigation — orbit, pan, zoom

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