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-core/src/abi.rs

@@ -891,6 +891,9 @@ pub unsafe extern "C" fn punktfunk_connect_ex4(
             pref,
             gamepad,
             bitrate_kbps,
+            // 8-bit only over the C ABI for now — the ABI doesn't yet carry the embedder's video
+            // caps (Apple/Android decode 8-bit). The native Windows client advertises 10-bit/HDR.
+            0,
             launch,
             pin,
             identity,