feat(linux/vulkan-encode): opt-in gamescope producer-native NV12 encode source

With PUNKTFUNK_PIPEWIRE_NV12=1 (bring-up gate), the PipeWire negotiation
offers an NV12 LINEAR DMA-BUF pod (BT.709 limited pinned MANDATORY) ahead
of the BGRx one, and gamescope's producer-side RGB->YUV pass replaces the
host CSC entirely: the encoder imports the two-plane buffer as a profiled
VIDEO_ENCODE_SRC image and the VCN encodes it directly. Contributed
measurements: encode p99 2.9 ms (from ~4-4.5 ms via EFC RGB-direct),
60 fps capture, 0 send drops.

Hardening on top of the contributed patch:
- unaligned modes (1080p!) stage through a padded aligned NV12 copy (edge
  rows/columns duplicated, transfer-only) instead of direct-importing the
  visible-size buffer -- a direct import would make the VCN read past the
  producer allocation, the exact OOB class behind the 2026-07-20 field GPU
  reset; the encode extents return to the aligned coded extent everywhere
- the UV plane layout honors the producer's plane-1 chunk (offset/stride)
  when the SPA buffer carries one (same-BO verified by inode), with the
  contiguous-plane contract as fallback
- PyroWave sessions are excluded from the gate (their Vulkan compute CSC
  ingests packed RGB), and a native-NV12 session that resolves to libav
  VAAPI (H264 codec, PUNKTFUNK_VULKAN_ENCODE=0, feature off, or a failed
  Vulkan open) refuses at open instead of streaming garbage chroma
- pad staging images carry TRANSFER_SRC (the width-padding pass self-copies
  the staging image -- previously missing on 1366-wide modes)
- metadata-cursor one-shot warn (parity with RGB-direct) and a padded-NV12
  PUNKTFUNK_PERF split label; the padded RGB copy gets its timestamps too

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

crates/pf-encode/src/lib.rs

@@ -330,6 +330,7 @@ fn open_video_backend(
             {
                 match vulkan_video::VulkanVideoEncoder::open(
                     codec,
+                    format,
                     width,
                     height,
                     fps,
@@ -344,12 +345,32 @@ fn open_video_backend(
                         );
                         return Ok((Box::new(e) as Box<dyn Encoder>, "vulkan"));
                     }
+                    // Native NV12 (PUNKTFUNK_PIPEWIRE_NV12 capture) has no VAAPI fallback:
+                    // libav's dmabuf lane would import the two-plane buffer as packed RGB
+                    // (silent garbage) and its CPU lane bails per frame — die crisply instead.
+                    Err(e) if format == PixelFormat::Nv12 => {
+                        return Err(e.context(
+                            "Vulkan Video open failed on a native-NV12 capture \
+                             (PUNKTFUNK_PIPEWIRE_NV12=1) — no VAAPI fallback exists; unset the \
+                             flag to restore the packed-RGB pipeline",
+                        ));
+                    }
                     Err(e) => tracing::warn!(
                         error = %format!("{e:#}"),
                         "Vulkan Video encode open failed — falling back to libav VAAPI"
                     ),
                 }
             }
+            // Same rule when the Vulkan backend was never eligible (H264 session,
+            // PUNKTFUNK_VULKAN_ENCODE=0, or a build without the feature).
+            if format == PixelFormat::Nv12 {
+                anyhow::bail!(
+                    "native NV12 capture (PUNKTFUNK_PIPEWIRE_NV12=1) requires the Vulkan Video \
+                     encoder (HEVC/AV1 session, --features vulkan-encode, \
+                     PUNKTFUNK_VULKAN_ENCODE not 0) — this session resolved to libav VAAPI; \
+                     unset the flag"
+                );
+            }
             vaapi::VaapiEncoder::open(
                 codec,
                 format,
@@ -390,6 +411,7 @@ fn open_video_backend(
                     }
                     vulkan_video::VulkanVideoEncoder::open(
                         codec,
+                        format,
                         width,
                         height,
                         fps,