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,

crates/punktfunk-core/src/client.rs

@@ -196,6 +196,10 @@ impl NativeClient {
         compositor: CompositorPref,
         gamepad: GamepadPref,
         bitrate_kbps: u32,
+        // Client video capabilities advertised to the host (bitfield of quic::VIDEO_CAP_10BIT /
+        // VIDEO_CAP_HDR) — the host upgrades to a 10-bit / HDR encode only when the matching bit is
+        // set. 0 = the 8-bit BT.709 stream every client understands.
+        video_caps: u8,
         launch: Option<String>,
         pin: Option<[u8; 32]>,
         identity: Option<(String, String)>,
@@ -245,6 +249,7 @@ impl NativeClient {
                     compositor,
                     gamepad,
                     bitrate_kbps,
+                    video_caps,
                     launch,
                     pin,
                     identity,
@@ -569,6 +574,7 @@ struct WorkerArgs {
     compositor: CompositorPref,
     gamepad: GamepadPref,
     bitrate_kbps: u32,
+    video_caps: u8,
     launch: Option<String>,
     pin: Option<[u8; 32]>,
     identity: Option<(String, String)>,
@@ -597,6 +603,7 @@ async fn worker_main(args: WorkerArgs) {
         compositor,
         gamepad,
         bitrate_kbps,
+        video_caps,
         launch,
         pin,
         identity,
@@ -657,10 +664,10 @@ async fn worker_main(args: WorkerArgs) {
                 name: None,
                 // Library id to launch this session, if the embedder asked for one.
                 launch: launch.clone(),
-                // TODO(hdr): advertise the embedder's real decode caps once the ABI carries them
-                // and the Apple/Linux clients decode 10-bit. 0 = 8-bit only — the host then never
-                // upgrades this connector's session to a stream it can't yet present.
-                video_caps: 0,
+                // The embedder's decode/present caps (e.g. the Windows client advertises
+                // VIDEO_CAP_10BIT | VIDEO_CAP_HDR). The host only upgrades to a 10-bit / HDR encode
+                // when the matching bit is set, so `0` stays an 8-bit BT.709 stream.
+                video_caps,
             }
             .encode(),
         )