feat(clients): HDR Steps 2-3 — apply mastering metadata + display capability-gate

Continues docs/hdr-pipeline-plan.md. Steps 0/1 + Step 2 (Windows/Android) already
landed in 3526517; this is Step 2 (Apple) + Step 3 (all clients). Client-only — no
core/host/ABI change (the 0xCE/next_hdr_meta/color_info surfaces shipped in Step 0).

Step 2 — clients APPLY the host's HDR metadata (each remaps from the wire form: ST.2086
G,B,R order, mastering luminance in 0.0001 cd/m2):
- Apple: connect via punktfunk_connect_ex5 (resurrects the previously-dead HDR pipeline);
  nextHdrMeta/colorInfo wrappers + HdrMeta SEI-blob builders; the pump drains nextHdrMeta
  -> VideoDecoder.setHdrMeta -> CVBufferSetAttachment of MasteringDisplayColorVolume (24B
  BE) + ContentLightLevelInfo (4B BE) on each HDR pixel buffer (correct for the
  itur_2100_PQ layer; CAEDRMetadata avoided as ambiguous there).

Step 3 — capability-gate: advertise HDR caps ONLY when the display can present it, so an
SDR display gets a proper BT.709 stream instead of PQ it would mis-tone-map; an HDR
display self-tone-maps from the Step-1/2 mastering metadata.
- Windows: present::display_supports_hdr() (DXGI any IDXGIOutput6 colour space == G2084),
  ANDed with the user HDR setting in session.rs; logs the SDR drop.
- Apple: NSScreen.maximumExtendedDynamicRangeColorComponentValue>1 (macOS) /
  UIScreen.main.potentialEDRHeadroom>1 (iOS) in SessionModel.
- Android: Settings.displaySupportsHdr (Display.getHdrCapabilities HDR10/HDR10+) passed
  through a new hdr_enabled jboolean on nativeConnect; session.rs gates the caps.

Validation: Android native (incl. the jboolean gate) builds + clippy clean via cargo-ndk;
fmt clean. Windows (MSVC), Apple (Swift) and the Kotlin side are CI/on-glass validated —
not compilable on the Linux dev box. Deferred to the RTX box: mid-session Reconfigure
SDR-downgrade on monitor move, and confirming the host emits SDR for an SDR client off an
HDR desktop.

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

clients/windows/src/present.rs

@@ -619,6 +619,36 @@ fn blob_bytes(blob: &ID3DBlob) -> &[u8] {
     }
 }
 
+/// True if any attached display is currently in HDR (BT.2020 PQ) mode. The client advertises HDR
+/// caps only when this holds, so an SDR display gets a proper 8-bit BT.709 stream instead of PQ it
+/// would mis-tone-map (the washed-out/dark failure); an HDR display self-tone-maps from the
+/// mastering metadata. Coarse — checks ANY output, not the app's specific monitor; a mid-session
+/// monitor move to/from HDR is a follow-up (the `Reconfigure` downgrade).
+pub fn display_supports_hdr() -> bool {
+    unsafe {
+        let factory: IDXGIFactory1 = match CreateDXGIFactory1() {
+            Ok(f) => f,
+            Err(_) => return false,
+        };
+        let mut ai = 0u32;
+        while let Ok(adapter) = factory.EnumAdapters1(ai) {
+            ai += 1;
+            let mut oi = 0u32;
+            while let Ok(output) = adapter.EnumOutputs(oi) {
+                oi += 1;
+                if let Ok(o6) = output.cast::<IDXGIOutput6>() {
+                    if let Ok(desc) = o6.GetDesc1() {
+                        if desc.ColorSpace == DXGI_COLOR_SPACE_RGB_FULL_G2084_NONE_P2020 {
+                            return true;
+                        }
+                    }
+                }
+            }
+        }
+    }
+    false
+}
+
 /// Generic HDR10 mastering metadata: BT.2020 primaries + D65 white, a 1000-nit mastering display,
 /// MaxCLL 1000 / MaxFALL 400. The fallback used only until the host's real `0xCE` metadata arrives.
 fn generic_hdr10_metadata() -> DXGI_HDR_METADATA_HDR10 {

clients/windows/src/session.rs

@@ -107,13 +107,19 @@ fn pump(
         params.compositor,
         params.gamepad,
         params.bitrate_kbps,
-        // Advertise 10-bit + HDR10 (when enabled): the presenter handles BT.2020 PQ frames (P010 on
-        // the GPU path, X2BGR10 on software), 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. A client that turns HDR off advertises `0` and always gets the 8-bit stream.
-        if params.hdr_enabled {
+        // Advertise 10-bit + HDR10 only when the user enabled HDR AND a display is actually in HDR
+        // mode: the host then upgrades HDR content to a Main10/PQ stream (its own 10-bit gate still
+        // applies). On an SDR display we advertise `0` so the host sends a proper 8-bit BT.709 stream
+        // rather than PQ the panel would mis-tone-map (washed-out/dark). An HDR display self-tone-maps
+        // from the mastering metadata we apply. The presenter handles BT.2020 PQ frames (P010 / X2BGR10).
+        if params.hdr_enabled && crate::present::display_supports_hdr() {
             punktfunk_core::quic::VIDEO_CAP_10BIT | punktfunk_core::quic::VIDEO_CAP_HDR
         } else {
+            if params.hdr_enabled {
+                tracing::info!(
+                    "HDR enabled in settings but no HDR display detected — requesting SDR"
+                );
+            }
             0
         },
         None, // launch: the Windows client has no library picker yet