fix(macos): compile the VideoToolbox import and clean platform warnings

The macOS job finally reached the build (after the vcpkg manifest fix) and
hit a macOS-only compile error in the VideoToolbox import: `*ptr as
*const T` parses as `(*ptr) as *const T`, so `sw_format` was read off a
pointer instead of the AVHWFramesContext. Bind the frames pointer first.

Also fix the warnings the cross-check surfaced: the redundant
MTLPixelFormat import, and doc comments on an extern block and a
thread_local! (rustdoc does not document those).

Verified locally with a host-cc wrapper:
`cargo check -p oak-core -p oak-codec -p oak-node -p oak-render
-p oak-task -p oak-plugin --target aarch64-apple-darwin` is clean.
(oak-app itself needs a real Apple toolchain for ring.)

crates/oak-codec/src/gpuinterop.rs

@@ -516,7 +516,7 @@ fn d3d11_import(req: &HwImportRequest<'_>, ctx: &Arc<GpuContext>) -> ImportOutco
 // macOS: VideoToolbox CVPixelBuffer -> IOSurface -> Metal textures
 // ---------------------------------------------------------------------------
 
-/// `CVPixelBufferGetIOSurface` (CoreVideo).
+// `CVPixelBufferGetIOSurface` (CoreVideo).
 #[cfg(target_os = "macos")]
 #[link(name = "CoreVideo", kind = "framework")]
 extern "C" {
@@ -543,7 +543,8 @@ fn videotoolbox_import(req: &HwImportRequest<'_>, ctx: &Arc<GpuContext>) -> Impo
 		return ImportOutcome::Unsupported;
 	}
 	// SAFETY: `hw_frames_ctx` is an AVHWFramesContext for this frame.
-	let sw_format = unsafe { (*(*frames).data as *const sys::AVHWFramesContext).sw_format };
+	let hw_frames = unsafe { (*frames).data as *const sys::AVHWFramesContext };
+	let sw_format = unsafe { (*hw_frames).sw_format };
 	let format = match sw_format {
 		sys::AVPixelFormat::AV_PIX_FMT_NV12 => PlanarFormat::Nv12,
 		sys::AVPixelFormat::AV_PIX_FMT_P010LE | sys::AVPixelFormat::AV_PIX_FMT_P010BE => {