feat(client/windows): HDR10 (BT.2020 PQ) decode + present

Light up the dormant 10-bit/HDR path end to end on the Windows client.

- core: NativeClient::connect gains a video_caps param threaded into the Hello. The Windows
  client advertises VIDEO_CAP_10BIT | VIDEO_CAP_HDR; every other caller (the C ABI shim,
  Linux, Android, host test connects) passes 0, so the 8-bit BT.709 path is unchanged. The
  host already gates a Main10/PQ encode on these bits + PUNKTFUNK_10BIT.
- video.rs: a PQ frame (color_trc == SMPTE2084) converts 10-bit YUV → X2BGR10 (== DXGI
  R10G10B10A2) with the BT.2020 matrix via sws_setColorspaceDetails; swscale applies only
  the matrix + range, so the PQ-encoded samples pass through untouched.
- present.rs: on an HDR frame the swapchain flips in place (ResizeBuffers) to R10G10B10A2 +
  DXGI_COLOR_SPACE_RGB_FULL_G2084_NONE_P2020 + HDR10 metadata; the passthrough shader is
  unchanged and the compositor maps PQ→display. Switched to ALPHA_MODE_IGNORE so the 10-bit
  padding bits don't render transparent. SDR stays 8-bit B8G8R8A8.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/punktfunk-client-windows/src/session.rs

@@ -30,7 +30,7 @@ pub struct SessionParams {
     pub identity: (String, String),
 }
 
-#[derive(Clone, Copy, Default)]
+#[derive(Clone, Copy, Default, PartialEq)]
 pub struct Stats {
     pub fps: f32,
     pub mbps: f32,
@@ -99,6 +99,10 @@ fn pump(
         params.compositor,
         params.gamepad,
         params.bitrate_kbps,
+        // Advertise 10-bit + HDR10: the presenter handles BT.2020 PQ (R10G10B10A2) frames, so the
+        // host may upgrade HDR content to a Main10/PQ stream (it still only does so for actual HDR
+        // content with its own 10-bit gate). 8-bit SDR is unaffected.
+        punktfunk_core::quic::VIDEO_CAP_10BIT | punktfunk_core::quic::VIDEO_CAP_HDR,
         None, // launch: the Windows client has no library picker yet
         params.pin,
         Some(params.identity),