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/project/Commands.h

@@ -24,7 +24,7 @@
 #include <QString>
 
 class QWidget;
-class ViewportWindow;
+class WgpuViewportWindow;
 namespace bonsaiviewer { class SessionState; }
 
 namespace bonsaiviewer::modules::project::commands {
@@ -32,21 +32,21 @@ namespace bonsaiviewer::modules::project::commands {
 // User-facing commands. Each owns its own dialogs and confirmations; each
 // emits exactly one notify() at the end (projectReset / projectOpened /
 // projectSaved) so views refresh once per command.
-bool newProject(SessionState& s, QWidget& host, ViewportWindow& vp);
-bool openProject(SessionState& s, QWidget& host, ViewportWindow& vp);
+bool newProject(SessionState& s, QWidget& host, WgpuViewportWindow& vp);
+bool openProject(SessionState& s, QWidget& host, WgpuViewportWindow& vp);
 // Open a specific .ifcfed by path, bypassing the file dialog. Used by the
 // "Open Recent" menu. Same dirty-check / load / cloud-resolve flow as
 // openProject; returns false if the load failed or was cancelled.
-bool openProjectPath(SessionState& s, QWidget& host, ViewportWindow& vp, const QString& path);
+bool openProjectPath(SessionState& s, QWidget& host, WgpuViewportWindow& vp, const QString& path);
 // Pick a connector, then call pull_ifcfed_interactive and open the resulting
 // .ifcfed as a fresh project. Non-local models in the loaded federation are
 // resolved asynchronously via pull_models.
-bool openCloudProject(SessionState& s, QWidget& host, ViewportWindow& vp);
+bool openCloudProject(SessionState& s, QWidget& host, WgpuViewportWindow& vp);
 // pull_ifcfed using the current project's .ifcfed.manifest. Re-downloads
 // the .ifcfed from the same cloud target it came from (typically without
 // user interaction), then opens it like a fresh project — discarding any
 // local edits after the usual dirty-check prompt.
-bool syncCloudProject(SessionState& s, QWidget& host, ViewportWindow& vp);
+bool syncCloudProject(SessionState& s, QWidget& host, WgpuViewportWindow& vp);
 bool saveProject(SessionState& s, QWidget& host);
 bool saveProjectAs(SessionState& s, QWidget& host);
 // Push the current federation to the cloud target named in its manifest