refactor: merge ifcviewer-wgpu into ifcviewer, drop Wgpu prefix

The GL backend is gone (task #53). The wgpu/non-wgpu folder split and
the Wgpu* class prefix were both disambiguation artefacts from the
overlap period — now pure dead weight.

## Folder + library merge

* `src/ifcviewer-wgpu/`  → folded into `src/ifcviewer/` (git mv tracks
  every file as a rename so blame/log history survives).
* `src/ifcviewer-wgpu-minimal/` → `src/ifcviewer-minimal/` (the exe was
  already named `IfcViewerMinimal`; this just brings the folder + CMake
  target name into line).
* `src/ifcviewer-wgpu/tests/test_wgpu_{selection,visibility}.cpp` →
  `src/ifcviewer/tests/test_{selection,visibility}.cpp`, folded into
  the existing `add_ifcviewer_unit_test(...)` helper.
* The `IfcViewerWgpu` static library is dissolved — its sources become
  part of the unified `IfcViewer` static library, which now bundles
  scene/loader + renderer in one target. The pre-merge circular
  dependency (IfcViewer linking IfcViewerWgpu just to get the
  ViewportWindow.h include path that SceneLoader.h needs) goes away.
* The wgpu-native FetchContent block, the Cocoa/QuartzCore link on
  Apple, the OBJCXX-enabled `.mm` source, and the wgpu-native runtime
  install all move into `src/ifcviewer/CMakeLists.txt` unchanged.

## Type renames (Wgpu prefix dropped from every Wgpu* identifier)

  WgpuAreaMeasurement   → AreaMeasurement
  WgpuBufferPool        → BufferPool
  WgpuLengthMeasurement → LengthMeasurement
  WgpuMetalSurface      → MetalSurface
  WgpuModelGpuData      → ModelGpuData
  WgpuOverlayFrame      → OverlayFrame
  WgpuOverlayRenderer   → OverlayRenderer
  WgpuSectionPlane      → SectionPlane
  WgpuSelectionState    → SelectionState
  WgpuStreamingLoader   → StreamingLoader
  WgpuStreamingThread   → StreamingThread
  WgpuViewportWindow    → ViewportWindow
  WgpuVisibilityState   → VisibilityState

  CMake target IfcViewerWgpuMinimal → IfcViewerMinimal (exe name was
  already this since wgpu shipped as default).

Deliberately kept: `onWgpuLog` (wgpu-native log callback — names a
binding to an external API, not one of *our* types), and the WGPU*
enum/struct prefixes from wgpu-native's own headers. `WgpuMemProbe`
lives in the separate `src/wgpu-mem-probe/` standalone diagnostic
project and isn't touched.

## Include-path updates

Every `#include "../ifcviewer-wgpu/Wgpu<X>.h"` → `"../ifcviewer/<X>.h"`,
every in-directory `#include "Wgpu<X>.h"` → `"<X>.h"`. Includes from
sibling subdirectories (modules/, etc.) are updated to point at
`../../../ifcviewer/` instead of `../../../ifcviewer-wgpu/`.

## cmake/CMakeLists.txt simplification

The redundant `add_subdirectory(ifcviewer-wgpu)` blocks (one inside
the BUILD_BONSAIVIEWER fan-in, one in the BONSAIVIEWER-less standalone
block) collapse into a single unconditional
`add_subdirectory(../src/ifcviewer ifcviewer)`. The standalone block
keeps only `wgpu-mem-probe` (the diagnostic tool, unrelated to the
viewer lib).

## Verification

* Full build green: `IfcViewer` static lib, `IfcViewerMinimal` exe,
  `BonsaiViewer` exe, all four pre-existing ifcviewer unit tests, and
  the two new-location tests (`test_selection`, `test_visibility`).
* No stray `Wgpu<X>` identifier remains across `src/ifcviewer/`,
  `src/bonsaiviewer/`, `src/ifcviewer-minimal/` (verified by grep).
* Renames tracked by git as `R` entries — `git log --follow` on
  ViewportWindow.cpp etc. continues to show history through the move.

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

src/bonsaiviewer/modules/viewport/Commands.cpp

@@ -22,11 +22,11 @@
 
 #include "../../SessionState.h"
 #include "../../../ifcviewer/Federation.h"
-#include "../../../ifcviewer-wgpu/WgpuViewportWindow.h"
+#include "../../../ifcviewer/ViewportWindow.h"
 
 namespace bonsaiviewer::modules::viewport::commands {
 
-void setHome(SessionState& session, WgpuViewportWindow& vp) {
+void setHome(SessionState& session, ViewportWindow& vp) {
     auto camera = vp.cameraState();
     Federation::HomeView home_view;
     home_view.target = camera.target;
@@ -37,7 +37,7 @@ void setHome(SessionState& session, WgpuViewportWindow& vp) {
     session.setStatusMessage("Camera", "Home view updated");
 }
 
-void goHome(SessionState& session, WgpuViewportWindow& vp) {
+void goHome(SessionState& session, ViewportWindow& vp) {
     Federation* federation = session.federation();
     if (!federation->hasHomeView()) {
         session.setStatusMessage("Camera", "No home view set for this project");
@@ -50,52 +50,52 @@ void goHome(SessionState& session, WgpuViewportWindow& vp) {
     session.setStatusMessage("Camera", "Home view restored");
 }
 
-void viewSelected(WgpuViewportWindow& vp) {
+void viewSelected(ViewportWindow& vp) {
     vp.focusOnSelectedObject();
 }
 
-void fly(SessionState& session, WgpuViewportWindow& vp) {
+void fly(SessionState& session, ViewportWindow& vp) {
     vp.requestActivate();
     vp.enterFpsMode();
     session.setStatusMessage("Mode", "Fly mode active");
 }
 
-void toggleSection(SessionState& session, WgpuViewportWindow& vp) {
+void toggleSection(SessionState& session, ViewportWindow& vp) {
     vp.toggleSectionTool();
     session.setStatusMessage("Section",
         vp.sectionToolActive() ? "Section tool active" : "Section tool off");
 }
 
-void clearSection(SessionState& session, WgpuViewportWindow& vp) {
+void clearSection(SessionState& session, ViewportWindow& vp) {
     vp.clearSectionPlanes();
     session.setStatusMessage("Section", "Section planes cleared");
 }
 
-void toggleDistance(WgpuViewportWindow& vp) {
+void toggleDistance(ViewportWindow& vp) {
     vp.toggleLengthTool();
 }
 
-void toggleArea(WgpuViewportWindow& vp) {
+void toggleArea(ViewportWindow& vp) {
     vp.toggleAreaTool();
 }
 
-void toggleVolume(WgpuViewportWindow& vp) {
+void toggleVolume(ViewportWindow& vp) {
     vp.toggleVolumeTool();
 }
 
-void hideSelected(WgpuViewportWindow& vp) {
+void hideSelected(ViewportWindow& vp) {
     vp.hideSelectedElements();
 }
 
-void isolateSelected(WgpuViewportWindow& vp) {
+void isolateSelected(ViewportWindow& vp) {
     vp.isolateSelectedElements();
 }
 
-void showAll(WgpuViewportWindow& vp) {
+void showAll(ViewportWindow& vp) {
     vp.showAllElements();
 }
 
-void invertVisibility(WgpuViewportWindow& vp) {
+void invertVisibility(ViewportWindow& vp) {
     vp.invertElementVisibility();
 }