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>

crates/punktfunk-host/src/encode.rs

@@ -873,18 +873,20 @@ fn nvenc_direct_enabled() -> bool {
         .unwrap_or(true)
 }
 
-/// Whether the raw Vulkan Video HEVC encode backend is active for AMD/Intel. **Opt-in for now**
-/// (design/linux-vulkan-video-encode.md) — `PUNKTFUNK_VULKAN_ENCODE=1` (also `true`/`yes`/`on`)
-/// selects it over libav VAAPI for an HEVC session; it gives real reference-frame invalidation
-/// (a clean P-frame recovery anchor via explicit DPB reference slots) that the libavcodec VAAPI
-/// path can't express. Will flip to default-on after on-glass validation, like
-/// [`nvenc_direct_enabled`]. Only consulted with `--features vulkan-encode`; a failed open falls
-/// back to VAAPI, so this can only improve recovery, never break a stream.
+/// Whether the raw Vulkan Video HEVC encode backend is active for AMD/Intel. **Default ON** —
+/// on-glass validated 2026-07-12 on an AMD RADV 780M with a real Deck-class client: the pipelined
+/// encoder ran a rock-solid 1080p@240 HEVC session and healed loss with clean P-frame recovery
+/// anchors (never IDR) via explicit DPB reference slots — real reference-frame invalidation the
+/// libavcodec VAAPI path can't express (design/linux-vulkan-video-encode.md). `PUNKTFUNK_VULKAN_ENCODE=0`
+/// (also `false`/`no`/`off`) is the libav-VAAPI escape hatch. Only consulted with
+/// `--features vulkan-encode`, and a failed open falls back to VAAPI, so an unsupported device
+/// (e.g. a Mesa without h265 encode, or an untested Intel/ANV where the path misbehaves at open)
+/// degrades gracefully to the old backend rather than breaking the stream.
 #[cfg(all(target_os = "linux", feature = "vulkan-encode"))]
 fn vulkan_encode_enabled() -> bool {
     std::env::var("PUNKTFUNK_VULKAN_ENCODE")
-        .map(|v| matches!(v.trim(), "1" | "true" | "yes" | "on"))
-        .unwrap_or(false)
+        .map(|v| !matches!(v.trim(), "0" | "false" | "no" | "off"))
+        .unwrap_or(true)
 }
 
 /// Cheap, side-effect-free NVIDIA-presence probe for the `auto` backend selector: the NVIDIA

crates/punktfunk-host/src/encode/linux/vulkan_video.rs

@@ -24,10 +24,11 @@ const IMPORT_CACHE_CAP: usize = 16;
 const CSC_SPV: &[u8] = include_bytes!("rgb2yuv.spv");
 /// DPB ring depth (well under the RADV `maxDpbSlots=17`); also the RFI recovery window.
 const DPB_SLOTS: u32 = 8;
-/// In-flight frame ring: how many captures may have GPU work outstanding at once. 3 overlaps a
-/// frame's CSC+encode with the next capture (the throughput win) while keeping added latency tiny
-/// (backpressure kicks in at the 3rd unread frame). Distinct from `DPB_SLOTS` (reference pool).
-const RING_DEFAULT: usize = 3;
+/// In-flight frame ring: how many captures may have GPU work outstanding at once. 2 overlaps a
+/// frame's CSC+encode with the next capture (the throughput win) at the lowest possible added
+/// latency — on-glass validated as rock-solid at 1080p@240, so it is the real-time default;
+/// backpressure kicks in at the 2nd unread frame. Distinct from `DPB_SLOTS` (reference pool).
+const RING_DEFAULT: usize = 2;
 
 /// Resolve the in-flight ring depth: `PUNKTFUNK_VULKAN_INFLIGHT` (clamped 2..=6), else `RING_DEFAULT`.
 fn ring_depth() -> usize {