ifcviewer-wgpu: bring up macOS Metal surface (task #32)

Without this, BonsaiViewer.app launched on macOS would show a white
viewport for the same reason Windows did before d4d0c934b — the
`createSurface()` else branch fell through to "wgpu surface creation
not yet wired for this platform", `init()` returned false, and the
Qt window background was all the user saw.

Pieces:

- `WgpuMetalSurface_mac.{h,mm}`: tiny Objective-C++ bridge. Takes the
  NSView pointer that Qt's `winId()` returns on macOS, attaches a
  CAMetalLayer (using Qt's existing one when surfaceType is
  MetalSurface, attaching one ourselves as a defensive fallback),
  sets `contentsScale` from the window's backing scale factor so
  retina drawables come out at native resolution, and returns the
  layer as `void*`. The .mm keeps the Objective-C namespace pollution
  out of WgpuViewportWindow.cpp.

- `WgpuViewportWindow` ctor: `setSurfaceType(QSurface::MetalSurface)`
  on macOS so Qt backs the NSView with a CAMetalLayer at window
  creation; OpenGLSurface elsewhere as before.

- `WgpuViewportWindow::createSurface()`: new `#elif defined(Q_OS_MAC)`
  branch that fills a `WGPUSurfaceSourceMetalLayer` with the layer
  pointer from the bridge and hands it to `wgpuInstanceCreateSurface`.

- `CMakeLists.txt`: `enable_language(OBJCXX)` + the .mm file added to
  the source list on Apple, and links `-framework Cocoa` (NSView) +
  `-framework QuartzCore` (CAMetalLayer).

`winId()` on macOS returns the backing NSView*, not the NSWindow* —
that's the layer-bearing host wgpu-native expects.

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

src/ifcviewer-wgpu/WgpuMetalSurface_mac.h

@@ -0,0 +1,32 @@
+/**
+ * Objective-C++ bridge between WgpuViewportWindow (pure C++) and Cocoa /
+ * QuartzCore (Objective-C). Compiled only on macOS — see CMakeLists.txt.
+ *
+ * Qt's QWindow::winId() returns the backing NSView* (as a WId) on macOS;
+ * we need a CAMetalLayer attached to that view to hand to wgpu-native
+ * via WGPUSurfaceSourceMetalLayer. Doing that requires Objective-C, so
+ * the actual layer attach lives in WgpuMetalSurface_mac.mm.
+ */
+
+#ifndef WGPU_METAL_SURFACE_MAC_H
+#define WGPU_METAL_SURFACE_MAC_H
+
+#if defined(__APPLE__)
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/// Ensures the given NSView has a CAMetalLayer as its backing layer.
+/// Returns the CAMetalLayer pointer (`void*` so callers don't need to
+/// pull QuartzCore into their TU); the layer is owned by the NSView.
+/// Returns nullptr if `nsview_ptr` is null.
+void* wgpu_macos_attach_metal_layer(void* nsview_ptr);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif // __APPLE__
+
+#endif // WGPU_METAL_SURFACE_MAC_H