feat(encode): default-on the Linux Vulkan Video HEVC backend

On-glass validated 2026-07-12 on an AMD RADV 780M with a real Deck-class
client: the pipelined raw-Vulkan HEVC encoder ran a rock-solid 1080p@240
session and healed loss with clean P-frame recovery anchors (real RFI the
libav VAAPI path can't express). Ship it on by default, mirroring the NVENC
default-on.

- vulkan_encode_enabled() defaults ON; PUNKTFUNK_VULKAN_ENCODE=0 is the libav
  VAAPI escape hatch. A failed open still falls back to VAAPI, so a device
  without h265 Vulkan encode (or an untested Intel/ANV that misbehaves at open)
  degrades gracefully instead of breaking the stream.
- Ring depth defaults to 2 (one frame of overlap, lowest added latency — the
  on-glass-validated real-time setting); PUNKTFUNK_VULKAN_INFLIGHT still tunes it.
- Compile --features punktfunk-host/vulkan-encode into the arch/deb/rpm host
  builds (pure-Rust ash, no new lib / no link-time dep), alongside nvenc.

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

packaging/rpm/punktfunk.spec

@@ -180,7 +180,11 @@ export PUNKTFUNK_BUILD_VERSION="%{version}-%{release}"
 # at runtime (no link-time dep; __requires_exclude already drops libcuda), so the binary starts
 # driver-less; the encoder engages only on a CUDA frame (default on NVIDIA; PUNKTFUNK_NVENC_DIRECT=0
 # opts back to libav) — the `cuda` gate keeps AMD/Intel on VAAPI regardless.
-cargo build --release --locked --features punktfunk-host/nvenc \
+# --features punktfunk-host/vulkan-encode: the AMD/Intel twin — a raw VK_KHR_video_encode_h265 backend
+# with real RFI (clean P-frame recovery anchor via DPB reference slots; design/linux-vulkan-video-encode.md).
+# Pure Rust `ash` (no new lib / no link-time dep); default on for HEVC (PUNKTFUNK_VULKAN_ENCODE=0 opts
+# back to libav VAAPI), and a failed open falls back to VAAPI so unsupported devices degrade gracefully.
+cargo build --release --locked --features punktfunk-host/nvenc,punktfunk-host/vulkan-encode \
   -p punktfunk-host -p punktfunk-client-linux -p punktfunk-client-session -p punktfunk-tray
 
 %if %{with web}