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>

docs-site/content/docs/roadmap.md

@@ -24,7 +24,7 @@ see [Status & Progress](/docs/status).
 | Web console + pairing | ✅ |
 | Concurrent sessions (shared desktop) | ✅ |
 | Network speed test + bitrate | ✅ |
-| HDR / 10-bit streaming | ✅ Windows host · ⛔ Linux host |
+| HDR / 10-bit streaming | ✅ Windows host · 🚧 Linux host (GNOME 50+ desktop mirror; virtual displays blocked upstream) |
 | Surround audio (5.1 / 7.1) | ✅ |
 | Sub-frame pipelining (latency) | 🔭 |
 
@@ -100,10 +100,14 @@ see [Status & Progress](/docs/status).
 
 ## ⛔ Parked / blocked
 
-- **HDR / 10-bit on the *Linux* host.** HDR streaming already works from a
-  [Windows host](/docs/windows-host) to an HDR-capable client (Windows, Android). On Linux it's
-  blocked upstream — no shipping compositor emits a 10-bit/HDR capture stream yet — and ready the
-  moment one does.
+- **HDR / 10-bit on the *Linux* host — virtual displays.** GNOME 50 added HDR screencasting for
+  **monitor** streams, and the host now uses it: the GameStream desktop mirror
+  (`PUNKTFUNK_VIDEO_SOURCE=portal`) negotiates the 10-bit PQ formats and encodes HEVC Main10
+  BT.2020 PQ (`punktfunk-host hdr-probe` reports readiness; on-glass validation pending). What
+  stays blocked upstream is HDR on **virtual monitors** — Mutter's `RecordVirtual` streams are
+  still SDR-only (through the GNOME 51 dev branch), so the native protocol and GameStream's
+  default virtual-display source stream 8-bit until that lands; the host is ready the moment it
+  does.
 - **Advanced DualSense voice-coil haptics.** Scoped and shelved (it rides the controller's USB audio
   interface, with near-zero game support on Linux). Adaptive triggers, rumble, and the lightbar
   already ship.