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_macos.md

@@ -66,7 +66,21 @@ brew install openimageio opencolorio openexr
 brew install portaudio expat
 ```
 
-### 7. Install Test Framework (Optional)
+### 7. Install Vulkan Backend Dependencies (Optional)
+
+Only needed for the Vulkan render backend. Oak builds the OpenGL backend regardless and falls back to it at runtime if Vulkan is unavailable:
+
+```bash
+brew install molten-vk vulkan-headers vulkan-loader
+```
+
+Point CMake at the loader so `find_package(Vulkan)` succeeds (add to `~/.zshrc` if you build regularly):
+
+```bash
+export VULKAN_SDK="$(brew --prefix vulkan-loader)"
+```
+
+### 8. Install Test Framework (Optional)
 
 Only needed if you plan to build and run tests: