Give the Steam Frame build its own passthrough default

SteamVR has no XR_FB_passthrough, so the Steam Frame build neither asks
for it nor offers the setting: [vr] passthrough defaults to false there,
and the F10/headset panel and the launcher hide the switch. The Frame
keeps the Android defaults that suit it (render scale 0.8, foveation
medium, the GX thread, object culling, performance level boost) and
starts at 120 Hz (previous commit).

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 +68,15 @@ int main() {
     Require(std::string_view(MKW_VR_REFRESH_RATE_DEFAULT_TEXT) == "0");
 #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());
+#if defined(MKW_HEADSET_STEAM_FRAME)
+    Require(!RuntimeConfigFile::kVrPassthroughDefault);
+#else
+    Require(RuntimeConfigFile::kVrPassthroughDefault);
+#endif
+
     // [vr] hand_tracking: the cockpit hands follow the headset's hand tracking.
     Require(Parse("[vr]\nhand_tracking = true\n").vrHandTracking == true);
     Require(Parse("[vr]\nhand_tracking = false\n").vrHandTracking == false);