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/Cargo.toml

@@ -0,0 +1,22 @@
+[package]
+name = "pf-ffvk"
+description = "Bindgen shim for FFmpeg's Vulkan hwcontext (libavutil/hwcontext_vulkan.h) — the AVVulkanDeviceContext/AVVkFrame surface ffmpeg-sys-next doesn't bind; enables Vulkan Video decode straight onto the presenter's device"
+version.workspace = true
+edition.workspace = true
+rust-version.workspace = true
+license.workspace = true
+authors.workspace = true
+repository.workspace = true
+
+# The bindings are generated at build time from the SYSTEM headers (libavutil +
+# vulkan-headers), so they are ABI-exact for the installed FFmpeg — including the
+# FF_API_VULKAN_* deprecation gates that change AVVulkanDeviceContext's layout between
+# FFmpeg builds. This is deliberately not hand-transcribed.
+
+[target.'cfg(target_os = "linux")'.dependencies]
+ash = { version = "0.38", features = ["loaded"] }
+
+[build-dependencies]
+# Same bindgen configuration as ffmpeg-sys-next (runtime = dlopen libclang).
+bindgen = { version = "0.72", features = ["runtime"], default-features = false }
+pkg-config = "0.3"