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/ifcviewer/SceneLoader.h

@@ -35,13 +35,13 @@
 #include <vector>
 
 #include "Federation.h"
-#include "../ifcviewer-wgpu/WgpuViewportWindow.h"
-#include "../ifcviewer-wgpu/WgpuStreamingLoader.h"
+#include "../ifcviewer/ViewportWindow.h"
+#include "../ifcviewer/StreamingLoader.h"
 #include "GeometryStreamer.h"
 #include "SidecarBuilder.h"
 #include "SidecarCache.h"
 
-// Drives IFC file loading into a WgpuViewportWindow.  Owns the per-model
+// Drives IFC file loading into a ViewportWindow.  Owns the per-model
 // GeometryStreamer, the load queue, the sidecar read thread, and the
 // next-free object_id counter used to rebase cached models onto the
 // current session's ID space.
@@ -55,7 +55,7 @@
 class SceneLoader : public QObject {
     Q_OBJECT
 public:
-    explicit SceneLoader(WgpuViewportWindow* viewport, QObject* parent = nullptr);
+    explicit SceneLoader(ViewportWindow* viewport, QObject* parent = nullptr);
     ~SceneLoader();
 
     // Sidecar cache use is opt-in per direction. Embedders that don't care
@@ -165,7 +165,7 @@ private:
     void applySidecarData(uint32_t mid, StreamingSidecar metadata);
     void startDataSourceLoad(uint32_t mid);
 
-    WgpuViewportWindow* viewport_ = nullptr;
+    ViewportWindow* viewport_ = nullptr;
     bool should_read_sidecar_ = false;
     bool should_write_sidecar_ = false;
     std::map<uint32_t, Model> models_;