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>

crates/pf-ffvk/wrapper.h

@@ -0,0 +1,3 @@
+/* The one header ffmpeg-sys-next's bindgen list omits: FFmpeg's Vulkan hwcontext.
+ * Pulls <vulkan/vulkan.h> (the vulkan-headers package) transitively. */
+#include <libavutil/hwcontext_vulkan.h>