ifcviewer-wgpu: pick the Windows ARM64 wgpu-native archive when targeting ARM64

The Windows branch of the wgpu-native FetchContent block was
hardcoding the x86_64 archive name regardless of host arch — the
macOS and Linux branches already switch on
\`CMAKE_SYSTEM_PROCESSOR MATCHES "arm64|aarch64"\`, but Windows
didn't get the same treatment because the wgpu work was done on
x86_64 hosts.

Result on \`windows-11-arm\`: CMake downloaded
\`wgpu-windows-x86_64-msvc-release.zip\`, IfcViewerWgpu linked against
the x86_64 import library, and the final link of
IfcViewerWgpuMinimal/BonsaiViewer emitted ~60 unresolved \`wgpu*\`
externs because the import-lib symbols are x86_64-only.

Upstream wgpu-native v29.0.0.0 already publishes
\`wgpu-windows-aarch64-msvc-release.zip\` — switching on
\`CMAKE_SYSTEM_PROCESSOR\` so the right archive gets fetched is
sufficient.

Also restores the ARM64 row in \`.github/workflows/build_win.yml\`
that the prior commit dropped (the comment there was wrong; upstream
does ship the binary, our CMake just wasn't asking for it).

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

src/ifcviewer-wgpu/CMakeLists.txt

@@ -32,7 +32,11 @@ set(WGPU_NATIVE_VERSION "v29.0.0.0" CACHE STRING "wgpu-native release tag")
 
 # Pick the right release archive for the host platform.
 if(CMAKE_SYSTEM_NAME STREQUAL "Windows")
-    set(_wgpu_archive "wgpu-windows-x86_64-msvc-release.zip")
+    if(CMAKE_SYSTEM_PROCESSOR MATCHES "ARM64|aarch64")
+        set(_wgpu_archive "wgpu-windows-aarch64-msvc-release.zip")
+    else()
+        set(_wgpu_archive "wgpu-windows-x86_64-msvc-release.zip")
+    endif()
     set(_wgpu_lib     "wgpu_native.dll.lib")
     set(_wgpu_runtime "wgpu_native.dll")
 elseif(CMAKE_SYSTEM_NAME STREQUAL "Darwin")