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/tests/vr_config_tests.cpp

@@ -68,6 +68,17 @@ int main() {
     Require(std::string_view(MKW_VR_REFRESH_RATE_DEFAULT_TEXT) == "0");
 #endif
 
+    // [vr] eye_tracked_foveation: the foveation centre follows the gaze, on the Steam Frame by default.
+    Require(Parse("[vr]\neye_tracked_foveation = true\n").vrEyeTrackedFoveation == true);
+    Require(Parse("[vr]\neye_tracked_foveation = false\n").vrEyeTrackedFoveation == false);
+    Require(!Parse("[vr]\neye_tracked_foveation = 1\n").vrEyeTrackedFoveation.has_value());
+    Require(!Parse("[vr]\n").vrEyeTrackedFoveation.has_value());
+#if defined(MKW_HEADSET_STEAM_FRAME)
+    Require(RuntimeConfigFile::kVrEyeTrackedFoveationDefault);
+#else
+    Require(!RuntimeConfigFile::kVrEyeTrackedFoveationDefault);
+#endif
+
     // [vr] passthrough: Horizon OS's room view, which the Steam Frame build does not offer.
     Require(Parse("[vr]\npassthrough = false\n").vrPassthrough == false);
     Require(!Parse("[vr]\n").vrPassthrough.has_value());