feat(hdr): GNOME 50 HDR screencast capture + Linux Main10 encode

GNOME 50 (Mutter MR 4928, PipeWire >= 1.6) added HDR screen sharing for
monitor streams: 10-bit PQ formats (xRGB_210LE/xBGR_210LE) with MANDATORY
BT.2020 + SMPTE-2084 colorimetry props, advertised while the mirrored
monitor is in BT.2100 colour mode. Wire the Linux host into it end-to-end
on the GameStream desktop-mirror path (PUNKTFUNK_VIDEO_SOURCE=portal):

* pf-frame: PixelFormat::X2Rgb10/X2Bgr10 (DRM XR30/XB30; X2Bgr10 is the
  Windows Rgb10a2 layout) + fourccs.
* pf-capture: want_hdr portal offer — HDR-only LINEAR-dmabuf pods with
  MANDATORY PQ/BT.2020 props (SHM excluded: Mutter's SHM record path
  paints 8-bit ARGB32 regardless of format; tiled excluded: the EGL
  de-tile blit is 8-bit RGBA8), negotiated-colorimetry parse, generic
  HDR10 hdr_meta(), packed-10-bit CPU cursor blend, a process-wide SDR
  downgrade latch on negotiation timeout, and a DisplayConfig BT.2100
  colour-mode probe (gnome_hdr_monitor_active).
* pf-encode: libav NVENC X2RGB10->P010 swscale (BT.2020 limited) ->
  HEVC Main10 / 10-bit AV1 with PQ VUI; VAAPI 10-bit on both paths (CPU
  P010 upload + dmabuf XR30 scale_vaapi p010/bt2020); can_encode_10bit
  now probes for real on Linux; 10-bit sessions route around the
  8-bit-only Vulkan-video/direct-NVENC backends.
* GameStream: host_hdr_capable() Linux arm, live monitor-HDR check at
  RTSP honor time, capturer-pool reuse keyed on HDR-ness, gs_bit_depth
  covers the new formats. New `punktfunk-host hdr-probe` diagnostic and
  a PUNKTFUNK_SPIKE_HDR spike lever.
* Native plane stays honestly 8-bit via capturer_supports_hdr(): Mutter
  RecordVirtual streams are SDR-only upstream (GNOME 50 and 51-dev), so
  virtual-display sources cannot deliver HDR yet.

Validated on the RTX 5070 Ti (GNOME 50.3 / PipeWire 1.6.8): the Main10
probes pass and the ignored nvenc_hdr10_smoke GPU test emits an IDR that
ffprobe reads as Main 10 / yuv420p10le / bt2020nc / smpte2084 / limited.
Live HDR capture negotiation still needs an HDR monitor on glass; VAAPI
10-bit needs the AMD box.

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

crates/pf-frame/src/lib.rs

@@ -56,6 +56,19 @@ pub enum PixelFormat {
     /// `DeviceBuffer::yuv444` — three full-res planes stacked in one allocation); NVENC encodes
     /// it natively under the Range-Extensions profile. Never a CPU payload.
     Yuv444,
+    /// 10-bit RGB packed `x:R:G:B 2:10:10:10` little-endian (SPA `xRGB_210LE`, DRM `XRGB2101010` /
+    /// `XR30`, ffmpeg `x2rgb10le`, NVENC `ARGB10`) — as an LE u32: B in bits 0-9, G 10-19, R 20-29.
+    /// The Linux GNOME 50+ HDR screencast source format: Mutter advertises it (with BT.2020
+    /// primaries + SMPTE ST.2084 PQ transfer) for a monitor in HDR mode, so the samples are
+    /// PQ-encoded BT.2020 RGB. Linux-only; the Windows HDR path stays `Rgb10a2`/`P010`.
+    X2Rgb10,
+    /// 10-bit RGB packed `x:B:G:R 2:10:10:10` little-endian (SPA `xBGR_210LE`, DRM `XBGR2101010` /
+    /// `XB30`, ffmpeg `x2bgr10le`, NVENC `ABGR10`) — as an LE u32: R in bits 0-9, G 10-19, B 20-29;
+    /// the same memory layout as the Windows [`Rgb10a2`](Self::Rgb10a2) (DXGI `R10G10B10A2`). The
+    /// second GNOME 50+ HDR screencast format (same PQ/BT.2020 colorimetry as
+    /// [`X2Rgb10`](Self::X2Rgb10)); kept separate from `Rgb10a2` so the Linux and Windows HDR
+    /// paths stay independently greppable.
+    X2Bgr10,
 }
 
 impl PixelFormat {
@@ -67,6 +80,12 @@ impl PixelFormat {
             _ => 4,
         }
     }
+
+    /// True for the packed 10-bit RGB layouts a Linux HDR (BT.2020 PQ) capture negotiates —
+    /// the formats that make a session's encode bit depth 10 (HEVC Main10 / 10-bit AV1).
+    pub fn is_hdr_rgb10(self) -> bool {
+        matches!(self, PixelFormat::X2Rgb10 | PixelFormat::X2Bgr10)
+    }
 }
 
 /// DRM FourCC for a packed 32-bit format name (little-endian, e.g. `b"XR24"`).
@@ -86,6 +105,9 @@ pub fn drm_fourcc(format: PixelFormat) -> Option<u32> {
         Bgra => drm_fourcc_code(b"AR24"), // DRM_FORMAT_ARGB8888
         Rgbx => drm_fourcc_code(b"XB24"), // DRM_FORMAT_XBGR8888
         Rgba => drm_fourcc_code(b"AB24"), // DRM_FORMAT_ABGR8888
+        // The GNOME 50+ HDR screencast formats (packed 2:10:10:10, PQ/BT.2020).
+        X2Rgb10 => drm_fourcc_code(b"XR30"), // DRM_FORMAT_XRGB2101010
+        X2Bgr10 => drm_fourcc_code(b"XB30"), // DRM_FORMAT_XBGR2101010
         // 24-bit packed RGB/BGR have no straightforward dmabuf import here; use the CPU path.
         // Rgb10a2/Nv12/P010 are the Windows HDR / video-processor formats — never produced on
         // Linux; Yuv444 is OUR convert's OUTPUT, never a capture source format.