Route bonsai through wgpu; delete the GL backend

Bonsai now drives the wgpu viewport for both sidecar and direct-IFC
loads. The GL viewer and its supporting state classes are gone.

SceneLoader rewire:
- Takes WgpuViewportWindow* instead of ViewportWindow*.
- Sidecar path reads metadata only (readSidecarMetadataOnly) and hands
  the StreamingSidecar off to the new applyCachedModel. Field accesses
  inside applySidecarData go through .meta.
- Direct-IFC path uses the wgpu A-path (upload{Mesh,Instance}Chunk +
  finalizeModel). The applyLodExtension call is dropped — wgpu has no
  live LOD1 splice; LOD1 still lands in the on-disk sidecar for the
  next open.

Bonsai migration:
- ViewportWindow → WgpuViewportWindow across MainWindow, Measurement,
  SessionState, and every modules/*/{Commands,Panel,View}.{h,cpp} —
  116 sites total. Same s/OverlayRenderer::/WgpuOverlayRenderer::/
  rename, 12 sites.
- Includes flipped from ../ifcviewer/ViewportWindow.h to
  ../ifcviewer-wgpu/WgpuViewportWindow.h. OverlayRenderer.h include
  dropped (transitively reached via the viewport header).
- BonsaiViewer links IfcViewerWgpu in addition to IfcViewer for the
  duration of the migration; the GL-side IfcViewer also publicly links
  IfcViewerWgpu so SceneLoader can resolve WgpuViewportWindow.

GL backend deletion:
- src/ifcviewer/ViewportWindow.{cpp,h}, BvhAccel.*, OverlayRenderer.*,
  Selection.*, Visibility.* all gone.
- src/ifcviewer-minimal/ removed entirely (MinimalWindow drove the GL
  viewport).
- src/ifcviewer/tests: test_bvh_accel, test_selection, test_visibility
  removed. The first has no replacement (wgpu doesn't use a per-instance
  BVH); the latter two are ported separately. test_lod_builder,
  test_sidecar_cache, test_instanced_geometry, test_federation remain
  (backend-agnostic).
- IfcViewer's CMakeLists drops OpenGL, Qt::OpenGL, Qt::Widgets — none
  of the surviving translation units reach for them.

Build flag plumbing:
- BUILD_BONSAIVIEWER now auto-enables BUILD_BONSAIVIEWER_WGPU since
  SceneLoader requires the wgpu lib for its WgpuViewportWindow* arg.
- The wgpu subprojects add_subdirectory ahead of the GL one so
  IfcViewerWgpu exists when IfcViewer's link evaluates.
- src/ifcviewer-minimal subdir reference removed from cmake/CMakeLists.

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

src/ifcviewer/SceneLoader.h

@@ -35,12 +35,13 @@
 #include <vector>
 
 #include "Federation.h"
-#include "ViewportWindow.h"
+#include "../ifcviewer-wgpu/WgpuViewportWindow.h"
+#include "../ifcviewer-wgpu/WgpuStreamingLoader.h"
 #include "GeometryStreamer.h"
 #include "SidecarBuilder.h"
 #include "SidecarCache.h"
 
-// Drives IFC file loading into a ViewportWindow.  Owns the per-model
+// Drives IFC file loading into a WgpuViewportWindow.  Owns the per-model
 // GeometryStreamer, the load queue, the sidecar read thread, and the
 // next-free object_id counter used to rebase cached models onto the
 // current session's ID space.
@@ -54,7 +55,7 @@
 class SceneLoader : public QObject {
     Q_OBJECT
 public:
-    explicit SceneLoader(ViewportWindow* viewport, QObject* parent = nullptr);
+    explicit SceneLoader(WgpuViewportWindow* viewport, QObject* parent = nullptr);
     ~SceneLoader();
 
     // Sidecar cache use is opt-in per direction. Embedders that don't care
@@ -175,10 +176,10 @@ private:
     void connectStreamer(GeometryStreamer* streamer);
     void joinSidecarThread();
     void joinDataSourceThreads();
-    void applySidecarData(uint32_t mid, SidecarData data);
+    void applySidecarData(uint32_t mid, StreamingSidecar metadata);
     void startDataSourceLoad(uint32_t mid);
 
-    ViewportWindow* viewport_ = nullptr;
+    WgpuViewportWindow* viewport_ = nullptr;
     bool should_read_sidecar_ = false;
     bool should_write_sidecar_ = false;
     std::map<uint32_t, Model> models_;