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-encode/src/enc/linux/vulkan_video.rs

@@ -84,13 +84,14 @@ fn rgb_request() -> Option<bool> {
     }
 }
 
-/// `PUNKTFUNK_VULKAN_RGB_TRUE_EXTENT=1` (bring-up): at unaligned modes, replace RGB-direct's
-/// padded-copy staging with a direct import carrying the TRUE-SIZE source `codedExtent` — RADV
-/// derives nonzero VCN firmware padding from it, so the EFC is told the source lacks the
-/// alignment rows (see [`RgbDirect::true_extent`]). Off by default until the EFC front-end is
-/// proven to honor the padding like the YUV fetch path does.
+/// True-extent RGB-direct at unaligned modes (default ON; `PUNKTFUNK_VULKAN_RGB_TRUE_EXTENT=0`
+/// restores the padded-copy staging): direct-import the visible-size capture with the TRUE-SIZE
+/// source `codedExtent` — RADV derives nonzero VCN firmware padding from it, so the EFC is told
+/// the source lacks the alignment rows (see [`RgbDirect::true_extent`]). Guarded-tested on Van
+/// Gogh 2026-07-21 (kernel clean, and the fastest 1080p encode path measured); the EFC only
+/// exists on Mesa ≥ 26, where the `codedExtent`-driven `session_init` is guaranteed.
 fn rgb_true_extent_request() -> bool {
-    std::env::var("PUNKTFUNK_VULKAN_RGB_TRUE_EXTENT").as_deref() == Ok("1")
+    std::env::var("PUNKTFUNK_VULKAN_RGB_TRUE_EXTENT").as_deref() != Ok("0")
 }
 
 /// Live RGB-direct session config: the chroma-siting bits the session was created with
@@ -105,13 +106,13 @@ struct RgbDirect {
     /// with the edge rows/columns duplicated into the padding (transfer-only, no shader) and
     /// encoded from there. Aligned modes keep the true zero-copy import.
     padded: bool,
-    /// `PUNKTFUNK_VULKAN_RGB_TRUE_EXTENT=1` bring-up alternative to `padded` at unaligned
-    /// modes: direct-import the visible-size buffer and pass the TRUE-SIZE source
+    /// The default unaligned-mode source strategy (`PUNKTFUNK_VULKAN_RGB_TRUE_EXTENT=0` falls
+    /// back to `padded`): direct-import the visible-size buffer and pass the TRUE-SIZE source
     /// `codedExtent` — RADV then programs nonzero firmware padding from it (Mesa ≥ 24.2
     /// derives `session_init` padding from `srcPictureResource.codedExtent`; see
     /// [`VulkanVideoEncoder::native_nv12`]), telling the VCN the source lacks the alignment
-    /// rows. The session/SPS/DPB stay app-aligned. Field-proven for YUV fetches; the EFC RGB
-    /// front-end honoring the padding is what this gate exists to prove.
+    /// rows, which the hardware edge-extends internally. The session/SPS/DPB stay app-aligned.
+    /// Guarded-tested on Van Gogh (kernel clean; fastest 1080p path measured).
     true_extent: bool,
 }
 
@@ -696,7 +697,7 @@ impl VulkanVideoEncoder {
                     ) =>
                         "active(true-extent: unaligned mode, direct import with the true-size \
                          source codedExtent — RADV firmware padding covers the alignment rows; \
-                         PUNKTFUNK_VULKAN_RGB_TRUE_EXTENT bring-up)",
+                         PUNKTFUNK_VULKAN_RGB_TRUE_EXTENT=0 restores the padded copy)",
                     (_, _, Some(RgbDirect { padded: false, .. })) => "active",
                     (_, _, Some(RgbDirect { padded: true, .. })) =>
                         "active(padded-copy: mode is not 64x16-aligned — staging blit + edge \