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-minimal/CMakeLists.txt

@@ -1,37 +0,0 @@
-################################################################################
-#                                                                              #
-# This file is part of IfcOpenShell.                                           #
-#                                                                              #
-# IfcOpenShell is free software: you can redistribute it and/or modify         #
-# it under the terms of the Lesser GNU General Public License as published by  #
-# the Free Software Foundation, either version 3.0 of the License, or          #
-# (at your option) any later version.                                          #
-#                                                                              #
-# IfcOpenShell is distributed in the hope that it will be useful,              #
-# but WITHOUT ANY WARRANTY; without even the implied warranty of               #
-# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the                 #
-# Lesser GNU General Public License for more details.                          #
-#                                                                              #
-# You should have received a copy of the Lesser GNU General Public License     #
-# along with this program. If not, see <http://www.gnu.org/licenses/>.         #
-#                                                                              #
-################################################################################
-
-message("Running CMakeLists.txt in /src/ifcviewer-minimal")
-
-file(GLOB IFCVIEWER_MINIMAL_CPP_FILES ${CMAKE_CURRENT_SOURCE_DIR}/*.cpp)
-file(GLOB IFCVIEWER_MINIMAL_H_FILES ${CMAKE_CURRENT_SOURCE_DIR}/*.h)
-set(IFCVIEWER_MINIMAL_FILES ${IFCVIEWER_MINIMAL_CPP_FILES} ${IFCVIEWER_MINIMAL_H_FILES})
-
-add_executable(IfcViewerMinimal ${IFCVIEWER_MINIMAL_FILES})
-
-set_target_properties(IfcViewerMinimal PROPERTIES
-    AUTOMOC ON
-    WIN32_EXECUTABLE ON
-    MACOSX_BUNDLE ON
-)
-
-target_link_libraries(IfcViewerMinimal PRIVATE IfcViewer)
-
-install(TARGETS IfcViewerMinimal EXPORT ${IFCOPENSHELL_EXPORT_TARGETS})
-ifcopenshell_deploy_qt_runtime(IfcViewerMinimal)