Centre foveation on the player's gaze on headsets with eye tracking

With [vr] eye_tracked_foveation (on by default on the Steam Frame, off
elsewhere) the runtime asks for XR_EXT_eye_gaze_interaction. When the
system reports an eye tracker, OpenXRInput binds the gaze pose and
locates it for each packet's display time, in the space the eye views
are located in; vr/eye_gaze.h turns it into tangents of each eye's own
view, which AuroraStereoFrame now carries (appended, after the existing
prefix).

Aurora centres the eye's fragment density map on the gaze snapped to a
cell of two map texels (about 3 degrees). Each eye keeps up to 32 maps,
one per cell looked at, so a glance back reuses its map; a new map is
bound once its upload completes, and until then the eye keeps the map
it had. Without a tracked gaze (a blink, no tracker, the setting off)
foveation centres on the forward direction exactly as before: the
forward maps are byte-identical.

Also logs every extension the OpenXR runtime offers at startup, so the
first Steam Frame session shows what SteamVR's Android runtime has.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019HBRGKTE1GnN2ah8gcZKr3

runtime/include/runtime_config.h

@@ -92,6 +92,7 @@ struct RuntimeUserConfig {
     std::optional<bool> vrWheelHaptics;
     std::optional<std::string> vrPerformanceLevel;
     std::optional<std::string> vrFoveation;
+    std::optional<bool> vrEyeTrackedFoveation;
     std::optional<std::string> vrRecenterKey;
     std::optional<float> vrLeanBackDegrees;
     // F10 > Diagnostics: OpenXR pacing and presentation logging in console.log.
@@ -320,6 +321,16 @@ inline bool IsSupportedVrFoveation(std::string_view value) {
     return std::find(kVrFoveationLevels.begin(), kVrFoveationLevels.end(), value) != kVrFoveationLevels.end();
 }
 
+// Eye-tracked foveation: with a headset that tracks the eyes (XR_EXT_eye_gaze_interaction, the Steam
+// Frame's), the foveation level's full-density region follows the gaze instead of staying on each
+// eye's forward direction. Live while the session's runtime offered the gaze at launch. On by
+// default on the Steam Frame; elsewhere off, since Horizon OS asks for an eye tracking permission.
+#if defined(MKW_HEADSET_STEAM_FRAME)
+inline constexpr bool kVrEyeTrackedFoveationDefault = true;
+#else
+inline constexpr bool kVrEyeTrackedFoveationDefault = false;
+#endif
+
 // The level aurora_set_stereo_foveation takes; anything unknown is off.
 inline uint32_t VrFoveationLevelIndex(std::string_view value) {
     const auto it = std::find(kVrFoveationLevels.begin(), kVrFoveationLevels.end(), value);
@@ -891,6 +902,7 @@ inline RuntimeUserConfig ParseConfigDocument(const toml::value& document) {
         value && IsSupportedVrFoveation(*value)) {
         config.vrFoveation = *value;
     }
+    config.vrEyeTrackedFoveation = FindConfigValue<bool>(document, "vr", "eye_tracked_foveation");
     if (auto value = FindConfigValue<std::string>(document, "vr", "mirror_view");
         value && IsSupportedVrMirrorView(*value)) {
         config.vrMirrorView = *value;
@@ -1303,6 +1315,11 @@ inline bool SetVrFoveation(std::string value) {
     return WriteSetting("vr", "foveation", FormatString(value));
 }
 
+inline bool SetVrEyeTrackedFoveation(bool value) {
+    Mutable().vrEyeTrackedFoveation = value;
+    return WriteSetting("vr", "eye_tracked_foveation", value ? "true" : "false");
+}
+
 inline bool SetVrFirstPersonSeat(std::string value) {
     if (!IsSupportedVrFirstPersonSeat(value)) {
         return false;
@@ -1828,6 +1845,10 @@ inline std::string VrFoveation(std::string fallback = kVrFoveationDefault) {
     return value && IsSupportedVrFoveation(*value) ? *value : std::move(fallback);
 }
 
+inline bool VrEyeTrackedFoveation(bool fallback = kVrEyeTrackedFoveationDefault) {
+    return Get().vrEyeTrackedFoveation.value_or(fallback);
+}
+
 inline int32_t VrFirstPersonHiddenModel(int32_t fallback = kVrFirstPersonHiddenModelDefault) {
     return std::clamp(Get().vrFirstPersonHiddenModel.value_or(fallback), -1, 31);
 }