feat(ffvk): bindgen shim for FFmpeg's Vulkan hwcontext (pf-ffvk)

ffmpeg-sys-next binds a curated header list that omits every
hwcontext_*.h, so AVVulkanDeviceContext/AVVulkanFramesContext/AVVkFrame
— the surface that lets FFmpeg's Vulkan Video decoder run on the
presenter's own VkDevice and hand the decoded VkImages back — had no
Rust bindings at all. pf-ffvk generates them at build time from the
SYSTEM headers, which is the only ABI-safe option: the struct layout
depends on FF_API_* deprecation gates resolved in libavutil/version.h
(confirmed: FFmpeg 8.1 still carries the FIXED_QUEUES fields). Ships
ash<->vulkan.h handle conversions; opaque AVHW*/AVFrame types keep
ffmpeg-sys-next the owner of the core surface (pointer casts across).
Needs vulkan-headers (Arch) / libvulkan-dev (CI, added);
PF_FFVK_VULKAN_INCLUDE overrides for cross builds. Verified: bindgen
layout tests pass, av_vk_frame_alloc links and zero-initializes.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

ci/rust-ci.Dockerfile

@@ -22,6 +22,8 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     libgl-dev libegl-dev libgbm-dev \
     # punktfunk-client-linux (GTK4/libadwaita shell, SDL3 gamepads)
     libgtk-4-dev libadwaita-1-dev libsdl3-dev \
+    # pf-ffvk (bindgen over libavutil/hwcontext_vulkan.h needs <vulkan/vulkan.h>)
+    libvulkan-dev \
     && rm -rf /var/lib/apt/lists/*
 
 # bun — builds the punktfunk-web console in deb.yml (which runs the web build in THIS image).