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/gamestream/rtsp.rs

@@ -396,18 +396,30 @@ fn stream_config(map: &HashMap<String, String>) -> Option<StreamConfig> {
         _ => Codec::H264,
     };
     // 10-bit/HDR request (Moonlight sets `dynamicRangeMode != 0` only when it both saw our Main10 SCM
-    // bit AND the user enabled HDR). Honor it only when the host can actually deliver Main10 (Windows +
-    // PUNKTFUNK_10BIT, `host_hdr_capable`); when honored, the video path proactively enables advanced
-    // color on the virtual display so a PQ stream flows even from an SDR desktop. A request we can't
-    // honor degrades to 8-bit SDR (and a desktop that is ALREADY HDR still streams PQ regardless, since
-    // the IDD-push capturer follows the display).
+    // bit AND the user enabled HDR). Honor it only when the host can actually deliver Main10
+    // (`host_hdr_capable` — Windows IDD-push, or the Linux GNOME 50+ portal mirror). On Windows,
+    // when honored, the video path proactively enables advanced color on the virtual display so a
+    // PQ stream flows even from an SDR desktop. On Linux the portal can only deliver PQ while the
+    // MIRRORED monitor is in HDR mode, so additionally probe the live colour mode here (one D-Bus
+    // round-trip, sync RTSP thread) — an SDR desktop honestly degrades to 8-bit SDR up front
+    // instead of running the capture negotiation into its timeout. A request we can't honor
+    // degrades to 8-bit SDR (and a Windows desktop that is ALREADY HDR still streams PQ
+    // regardless, since the IDD-push capturer follows the display).
     let hdr_requested = parse_u("x-nv-video[0].dynamicRangeMode").unwrap_or(0) != 0;
-    let hdr = hdr_requested && crate::gamestream::host_hdr_capable();
+    let mut hdr = hdr_requested && crate::gamestream::host_hdr_capable();
     if hdr_requested && !hdr {
         tracing::warn!(
             "client requested HDR (dynamicRangeMode != 0) but host is not HDR-capable — streaming 8-bit SDR"
         );
     }
+    #[cfg(target_os = "linux")]
+    if hdr && !pf_capture::gnome_hdr_monitor_active() {
+        tracing::warn!(
+            "client requested HDR but no monitor is in BT.2100 (HDR) colour mode — enable HDR in \
+             GNOME Settings → Displays (GNOME 50+) to stream it; streaming 8-bit SDR"
+        );
+        hdr = false;
+    }
     // The client's requested CSC (moonlight-common-c SdpGenerator.c: `encoderCscMode =
     // (colorspace << 1) | fullRange` — colorspace 0=Rec601, 1=Rec709, 2=Rec2020). Moonlight
     // renderers configure their YUV→RGB from this REQUESTED value (not the bitstream VUI), so a