packaging: ship the Vulkan backend on all platforms, standard lib layout

- Linux CD builds no longer disable Vulkan: the AppImage, deb, rpm and
  Arch packages now build and ship liboakvulkan (the AppImage deploys
  it via linuxdeploy --library so libvulkan is bundled too, and the
  verify step checks both backend libraries); deb/rpm dependencies gain
  libvulkan1/vulkan-loader; the Arch PKGBUILD gains vulkan-headers
- macOS CD installs vulkan-loader and exports VULKAN_SDK so
  find_package(Vulkan) locates the Homebrew loader (previously the
  Vulkan backend silently never built there)
- ffmpeg_bridge now installs to the standard lib directory with
  /../lib RPATH instead of the non-standard ffmpeg_bridge/bin
  layout (verified: editor, worker, oakgl and oakvulkan all resolve it)
- build guides (EN/ZH) document the macOS Vulkan backend dependencies
  and the VULKAN_SDK variable

docs/build.md

@@ -208,7 +208,7 @@ Install dependencies:
 
 ```bash
 brew update
-brew install cmake ninja pkg-config qt@6 ffmpeg openimageio opencolorio openexr portaudio expat molten-vk vulkan-headers
+brew install cmake ninja pkg-config qt@6 ffmpeg openimageio opencolorio openexr portaudio expat molten-vk vulkan-headers vulkan-loader
 ```
 
 Build OpenTimelineIO (optional, required for OTIO support):
@@ -231,6 +231,9 @@ export PATH="$(brew --prefix qt@6)/bin:$PATH"
 export CMAKE_PREFIX_PATH="$(brew --prefix qt@6)"
 export OTIO_LOCATION="${PWD}/otio-install"
 export OCIO_LOCATION="$(brew --prefix opencolorio)"
+# Let CMake's FindVulkan locate the Homebrew Vulkan loader (optional,
+# enables the Vulkan render backend)
+export VULKAN_SDK="$(brew --prefix vulkan-loader)"
 
 cmake -S . -B build -G Ninja -DBUILD_TESTS=ON -DBUILD_QT6=ON \
   -DOTIO_LOCATION="${OTIO_LOCATION}" \