feat(host/encode): VAAPI zero-copy dmabuf import (AMD/Intel GPU CSC)

Phase 2 of AMD/Intel support: the VAAPI encoder now takes the capture dmabuf
directly and does the RGB->NV12 colour conversion on the GPU's video engine,
eliminating the host-side de-pad + swscale CSC + upload the CPU path pays.

- capture: a vendor-neutral FramePayload::Dmabuf (dup'd fd + fourcc/modifier/
  layout). When zero-copy is on, the EGL->CUDA importer is unavailable (any
  non-NVIDIA host), and the backend is VAAPI, the capturer advertises LINEAR
  dmabuf and hands the raw buffer to the encoder instead of CPU-copying it.
- encode/vaapi: the encoder self-configures from the first frame's payload (no
  open_video signature change). The dmabuf arm wraps the buffer as an
  AV_PIX_FMT_DRM_PRIME frame and pushes it through a filter graph
  buffer(drm_prime) -> hwmap(vaapi) -> scale_vaapi=nv12 -> buffersink; the
  encoder takes NV12 surfaces straight from the sink. The Phase 1 CPU-upload
  path is kept as the other arm (used when capture produces CPU frames).

Live-validated on a Radeon 780M (real Sway/xdpw desktop capture): correct,
pixel-perfect HEVC, and ~10x less host CPU at 1440p (4.2s -> 0.4s of CPU for
300 frames) -- the de-pad/CSC/upload moves to the GPU. NVIDIA unchanged
(zero-copy still imports to CUDA; the passthrough path only engages on
non-NVIDIA hosts).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/punktfunk-host/src/encode.rs

@@ -304,6 +304,22 @@ fn nvidia_present() -> bool {
     std::path::Path::new("/dev/nvidiactl").exists() || std::path::Path::new("/dev/nvidia0").exists()
 }
 
+/// True if the Linux GPU encode backend resolves to VAAPI (AMD/Intel) rather than NVENC — mirrors
+/// [`open_video`]'s dispatch so the capturer can choose the matching zero-copy path (raw dmabuf
+/// passthrough for VAAPI vs the EGL→CUDA import for NVENC).
+#[cfg(target_os = "linux")]
+pub fn linux_zero_copy_is_vaapi() -> bool {
+    match std::env::var("PUNKTFUNK_ENCODER")
+        .unwrap_or_default()
+        .to_ascii_lowercase()
+        .as_str()
+    {
+        "nvenc" | "nvidia" | "cuda" => false,
+        "vaapi" | "amd" | "intel" => true,
+        _ => !nvidia_present(),
+    }
+}
+
 #[cfg(target_os = "linux")]
 mod linux;
 #[cfg(all(target_os = "windows", feature = "nvenc"))]