ci: fix bonsai cross-platform build on Linux, macOS arm64, Windows x64

Bundles four portability fixes uncovered by manually firing the platform
workflows against this branch:

- WgpuOverlayRenderer.cpp: GCC 11 (Rocky manylinux runner) does not parse
  a multi-line raw string inside `#define`. Converted
  THICK_LINE_HELPERS_WGSL from a `#define` to a `static const char*` and
  switched AXIS_WGSL / SECTION_WGSL / MARQUEE_WGSL to `std::string` so
  they can concatenate at static-init time. Three call sites now pass
  `.c_str()` to svFromCStr.

- bonsaiviewer/CMakeLists.txt: added BUNDLE DESTINATION to the install
  rule (same fix already applied to IfcViewerWgpuMinimal). MACOSX_BUNDLE
  targets fail at configure on macOS without it even when nobody runs
  `make install`.

- build_osx.yml: dropped the x64 (Intel cross-compile) matrix row. The
  runner is arm64 so `brew --prefix qt` returns the arm64 prefix; we'd
  need a separate x86_64 Qt install under /usr/local to cross-build
  BonsaiViewer. Revisit if Intel-Mac demand resurfaces.

- build_win.yml: dropped the ARM64 matrix row. wgpu-native does not ship
  a Windows-ARM64 binary, so IfcViewerWgpu's link step fails with ~60
  unresolved wgpu* externs. Re-enable when upstream publishes that
  target.

Cherry-pick this commit to v0.8.0 so the workflow_dispatch buttons see
the dropped rows.

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

.github/workflows/build_osx.yml

@@ -10,10 +10,11 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - os: macos
-            runner: macos-14
-            arch: x64
-            oldarch: 
+          # x64 (Intel cross-compile) dropped while wgpu Qt is required:
+          # the runner is arm64 so `brew --prefix qt` returns the arm64
+          # prefix; we'd need a separate x86_64 Qt install under
+          # /usr/local to cross-build BonsaiViewer. Revisit if Intel-Mac
+          # demand resurfaces.
           - os: macos
             runner: macos-14
             arch: arm64