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/punktfunk-host/src/main.rs

@@ -283,6 +283,24 @@ fn real_main() -> Result<()> {
         // PASS/FAIL + max Y/Cb/Cr error.
         #[cfg(target_os = "windows")]
         Some("hdr-p010-selftest") => crate::capture::dxgi::hdr_p010_selftest(),
+        // Linux HDR readiness probe (GNOME 50+ portal path): prints whether a monitor is currently
+        // in BT.2100 (HDR) colour mode, whether the NVENC/VAAPI backend probes Main10 for
+        // HEVC/AV1, and the GameStream HDR capability the two combine into — the "why isn't my
+        // stream HDR?" diagnostic (no display/session needed for the encoder half).
+        #[cfg(target_os = "linux")]
+        Some("hdr-probe") => {
+            let monitor_hdr = pf_capture::gnome_hdr_monitor_active();
+            let hevc10 = encode::can_encode_10bit(encode::Codec::H265);
+            let av110 = encode::can_encode_10bit(encode::Codec::Av1);
+            println!("monitor in BT.2100 (HDR) colour mode: {monitor_hdr}");
+            println!("encoder Main10 (HEVC): {hevc10}");
+            println!("encoder 10-bit (AV1):  {av110}");
+            println!(
+                "GameStream HDR capable (PUNKTFUNK_10BIT + video_source=portal + encoder): {}",
+                gamestream::host_hdr_capable()
+            );
+            Ok(())
+        }
         // Compositor readiness probe: exit 0 iff the (detected or PUNKTFUNK_COMPOSITOR-forced)
         // compositor is up and able to create a virtual output *now*. A session-bringup
         // script polls this to gate on real readiness instead of a blind `sleep`.