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/bonsaiviewer/modules/models/Commands.h

@@ -28,7 +28,7 @@
 #include <cstdint>
 
 class QWidget;
-class ViewportWindow;
+class WgpuViewportWindow;
 namespace bonsaiviewer { class SessionState; }
 
 namespace bonsaiviewer::modules::models::commands {
@@ -41,7 +41,7 @@ void renameGroup(SessionState& s, QWidget& host, const QString& group_id);
 void moveGroup(SessionState& s, const QString& id, const QString& parent_group_id);
 void moveModels(SessionState& s, const QStringList& ids, const QString& parent_group_id);
 void removeGroup(SessionState& s, QWidget& host, const QString& group_id);
-void removeModel(SessionState& s, ViewportWindow& vp, QWidget& host, const QString& fed_id);
+void removeModel(SessionState& s, WgpuViewportWindow& vp, QWidget& host, const QString& fed_id);
 void addModel(SessionState& s, QWidget& host);
 // Connector picker → pull_models_interactive → addCloudModel + load.
 // Reachable from AddModelDialog's CloudModel button; the underlying call