feat(linux/vulkan-encode): default-on native NV12 capture + RGB true-extent

Flip both zero-copy levers from opt-in to default:

- The capture negotiation now PREFERS gamescope's producer-side NV12 pod by
  default (PUNKTFUNK_PIPEWIRE_NV12=0 restores the packed-RGB negotiation).
  Codec-aware gating rides a new OutputFormat::nv12_native ->
  ZeroCopyPolicy::native_nv12_session edge resolved by the host facade from
  the session plan (pf_encode::linux_native_nv12_ok): only H265/AV1 sessions
  whose backend can open the raw Vulkan Video encoder ever see the NV12 pod
  -- an H264/Moonlight session (libav VAAPI, which would misread the
  two-plane buffer) keeps today's BGRx negotiation, as do the
  GameStream-resolve and portal-mirror paths, PyroWave, and NVENC prefs.
- RGB-direct's unaligned modes default to the true-extent direct import
  (PUNKTFUNK_VULKAN_RGB_TRUE_EXTENT=0 restores the padded-copy staging).
  Guarded-tested on Van Gogh with the kernel journal watched: clean, and at
  5.38 ms p50 the fastest 1080p encode path measured on that hardware. The
  EFC only exists on Mesa >= 26, where the codedExtent-driven session_init
  padding is guaranteed (verified back to Mesa 24.2).

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

crates/pf-capture/src/lib.rs

@@ -259,6 +259,12 @@ pub struct ZeroCopyPolicy {
     /// passthrough (like the VAAPI backend) instead of the EGL→CUDA import whose payloads only
     /// NVENC can consume. Per-session (the codec is negotiated), unlike `backend_is_vaapi`.
     pub pyrowave_session: bool,
+    /// THIS session's encoder can ingest a producer-native NV12 capture (the Linux raw Vulkan
+    /// Video backend on an H265/AV1 session — resolved by the host facade via
+    /// `pf_encode::linux_native_nv12_ok`). Gates whether the negotiation PREFERS gamescope's
+    /// producer-side NV12 pod: libav VAAPI (H264's backend) would misread the two-plane buffer,
+    /// so H264/GameStream/PyroWave sessions must never see NV12 frames.
+    pub native_nv12_session: bool,
     /// The PyroWave encoder's Vulkan-importable dmabuf modifiers for the capture's packed-RGB fourcc,
     /// resolved when the session encodes PyroWave (the passthrough advertises them so Mutter+NVIDIA,
     /// which allocates tiled-only, still negotiates zero-copy). Empty otherwise.