feat(client-linux): VAAPI hardware decode — zero-copy dmabuf into GraphicsOffload

Stage 1.5: on Intel/AMD clients libavcodec's VAAPI hwaccel decodes on
the GPU; frames map to DRM-PRIME dmabufs (av_hwframe_map, zero copy)
and reach GTK as GdkDmabufTexture (BT.709 limited CICP color state —
GDK's dmabuf default is BT.601). Inside GtkGraphicsOffload that is the
decoder-to-subsurface path, direct-scanout eligible when fullscreen.

Fallback ladder, live-verified on the NVIDIA dev box: no VAAPI device
-> software decode at session start (logged reason); a mid-session
VAAPI error (e.g. broken nvidia-vaapi-driver) demotes to software and
the host's IDR/RFI recovery resynchronizes; a rejected dmabuf import
logs and the stream continues. PUNKTFUNK_DECODER=software|vaapi
overrides; the first-frame log now names the active path.

The hwaccel path is raw ffmpeg-sys FFI (ffmpeg-next wraps none of it):
hw device ctx + get_format pinned to AV_PIX_FMT_VAAPI (NONE on
mismatch so cpu-fallback never silently engages inside libavcodec),
thread_count=1, LOW_DELAY. Surface lifetime rides DrmFrameGuard into
the texture's release func — GDK runs it on both success and failure.

Needs an Intel/AMD client box (Steam Deck/Bazzite) to live-verify the
hardware path; the software path is unchanged and revalidated.

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

crates/punktfunk-client-linux/src/session.rs

@@ -158,11 +158,11 @@ fn pump(
                     Ok(Some(decoded)) => {
                         total_frames += 1;
                         if total_frames == 1 {
-                            tracing::info!(
-                                width = decoded.width,
-                                height = decoded.height,
-                                "first frame decoded"
-                            );
+                            let (w, h, path) = match &decoded {
+                                DecodedFrame::Cpu(c) => (c.width, c.height, "software"),
+                                DecodedFrame::Dmabuf(d) => (d.width, d.height, "vaapi-dmabuf"),
+                            };
+                            tracing::info!(width = w, height = h, path, "first frame decoded");
                         }
                         // Latency: our wall clock expressed in the host's capture clock,
                         // minus the host-stamped capture pts (same math as client-rs).