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>

cmake/CMakeLists.txt

@@ -71,9 +71,18 @@ option(BUILD_EXAMPLES "Build example applications." ON)
 option(BUILD_GEOMSERVER "Build IfcGeomServer executable (Open CASCADE is required)." ON)
 option(BUILD_IFCMAX "Build IfcMax, a 3ds Max plug-in, Windows-only." OFF)
 option(BUILD_QTVIEWER "Build IfcOpenShell Qt GUI Viewer" OFF) # QtViewer requires Qt6
+option(BUILD_IFCMODEL_UI "Build minimal Qt IFC model UI prototype" OFF)
 option(BUILD_BONSAIVIEWER "Build Bonsai Viewer" OFF) # Requires Qt6 + OpenGL 4.5
 option(BUILD_BONSAIVIEWER_TESTS "Build unit tests for Bonsai Viewer core (fetches Catch2 v3)" OFF)
 option(BUILD_BONSAIVIEWER_WGPU "Build the experimental wgpu backend (fetches wgpu-native binary release)" OFF)
+# IfcViewer (the GL static lib) now links against IfcViewerWgpu because
+# SceneLoader drives the wgpu viewport. Auto-enable the wgpu subproject
+# whenever BUILD_BONSAIVIEWER is on so the link target exists.
+if(BUILD_BONSAIVIEWER AND NOT BUILD_BONSAIVIEWER_WGPU)
+    message(STATUS "BUILD_BONSAIVIEWER implies BUILD_BONSAIVIEWER_WGPU "
+                   "(SceneLoader uses WgpuViewportWindow); auto-enabling.")
+    set(BUILD_BONSAIVIEWER_WGPU ON)
+endif()
 option(BUILD_PACKAGE "" OFF)
 
 option(
@@ -645,6 +654,10 @@ if(BUILD_QTVIEWER)
     add_subdirectory(../src/qtviewer qtviewer)
 endif()
 
+if(BUILD_IFCMODEL_UI)
+    add_subdirectory(../src/ifcmodel-ui ifcmodel-ui)
+endif()
+
 if(BUILD_IFCGEOM)
     # install(FILES ${IFCGEOM_H_FILES}
     #     DESTINATION ${INCLUDEDIR}/ifcgeom
@@ -686,17 +699,20 @@ if(BUILD_BONSAIVIEWER)
         enable_testing()
     endif()
 
+    # Wgpu subprojects added first because IfcViewer's SceneLoader now
+    # links against IfcViewerWgpu; the target must exist when IfcViewer's
+    # CMakeLists runs.
+    if(BUILD_BONSAIVIEWER_WGPU)
+        add_subdirectory(../src/ifcviewer-wgpu ifcviewer-wgpu)
+        add_subdirectory(../src/ifcviewer-wgpu-minimal ifcviewer-wgpu-minimal)
+    endif()
     add_subdirectory(../src/ifcviewer ifcviewer)
-    add_subdirectory(../src/ifcviewer-minimal ifcviewer-minimal)
     add_subdirectory(../src/bonsaiviewer bonsaiviewer)
 endif()
 
-# The wgpu backend is gated independently of BUILD_BONSAIVIEWER: it shares
-# Qt but does not depend on the GL viewer's static lib, so a developer can
-# build it on its own to iterate on the port without compiling IfcGeom etc.
-# However most stages of the port will reference Federation/SceneLoader
-# headers from src/ifcviewer, so in practice it ships alongside.
-if(BUILD_BONSAIVIEWER_WGPU)
+# Standalone wgpu build (no GL viewer / no bonsai) still supported for
+# fast iteration on the port.
+if(BUILD_BONSAIVIEWER_WGPU AND NOT BUILD_BONSAIVIEWER)
     add_subdirectory(../src/ifcviewer-wgpu ifcviewer-wgpu)
     add_subdirectory(../src/ifcviewer-wgpu-minimal ifcviewer-wgpu-minimal)
     add_subdirectory(../src/wgpu-mem-probe wgpu-mem-probe)