Give the Steam Frame's native build the standalone headset defaults

MKW_VR_STANDALONE (runtime_config.h) names the standalone headsets: the
Quest, and the Steam Frame in its Android flavour and its native SteamOS
build. They share render scale 0.8, foveation medium, the GX thread,
the game's object culling, and the headset panel's foveation and
tracked-hands rows; passthrough stays Quest-only.

The Frame's native build also sets AuroraConfig::xrHeadsetOnly, so
Aurora neither presents the desktop window nobody watches nor renders
it past the last pass the eyes sample, which Android does always (4 to
6 ms of a 12 ms GPU frame on a Quest 3).

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

aurora-main/include/aurora/aurora.h

@@ -289,6 +289,10 @@ typedef struct {
   bool hasD3D12AdapterLuid;
   uint32_t d3d12AdapterLuidLow;
   int32_t d3d12AdapterLuidHigh;
+  // Nobody watches the desktop window while the headset runs (a standalone
+  // headset such as the Steam Frame's native build): skip presenting it and stop
+  // the mono render after the last pass the eyes sample, as Android always does.
+  bool xrHeadsetOnly;
 } AuroraConfig;
 
 typedef struct {

aurora-main/lib/aurora.cpp

@@ -1416,7 +1416,7 @@ bool headset_owns_display() noexcept {
 #if defined(__ANDROID__)
   return stereo_frame_provider_active();
 #else
-  return false;
+  return g_config.xrHeadsetOnly && stereo_frame_provider_active();
 #endif
 }