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/include/runtime_config.h

@@ -189,8 +189,9 @@ inline constexpr uint32_t kPostProcessingBloomPath = 0x10u;
 inline constexpr uint32_t kDisabledPostProcessingPathsDefault = kPostProcessingBloomPath;
 
 // Headset eye size as a fraction of what the OpenXR runtime recommends. A
-// standalone headset renders on a mobile GPU, so the Quest starts below it;
-// the launcher's first Config.toml (GameStorage.kt) writes the same value.
+// standalone headset renders on a mobile GPU, so the Quest and the Steam Frame
+// start below it; the launcher's first Config.toml (GameStorage.kt) writes the
+// same value.
 #if defined(__ANDROID__)
 inline constexpr float kVrRenderScaleDefault = 0.8f;
 #define MKW_VR_RENDER_SCALE_DEFAULT_TEXT "0.8"
@@ -306,7 +307,8 @@ inline bool IsSupportedVrRefreshRate(uint32_t value) {
 // 4x4 pixel blocks, the higher the level the closer to the centre. Whether the
 // GPU device gets fragment density maps at all is decided at launch, so going
 // from "off" to a level takes a restart; between levels and back to "off" it is
-// live. The Quest starts at "medium"; elsewhere it does nothing.
+// live. The standalone headsets (Quest, Steam Frame) start at "medium";
+// elsewhere it does nothing.
 #if defined(__ANDROID__)
 inline constexpr const char* kVrFoveationDefault = "medium";
 #else
@@ -1719,9 +1721,16 @@ inline bool SetVrRaceView(VrRaceView view) {
 
 // The room, through the headset's cameras, around the menu screen and every
 // other virtual screen, a Flat Screen race included, and around the immersive
-// window (never a fully immersive race). Only the Quest offers it; the
-// launcher's Settings page shows the same default.
-inline bool VrPassthrough(bool fallback = true) {
+// window (never a fully immersive race). Only the Quest offers it
+// (XR_FB_passthrough, which SteamVR does not have); the launcher's Settings
+// page shows the same default.
+#if defined(MKW_HEADSET_STEAM_FRAME)
+inline constexpr bool kVrPassthroughDefault = false;
+#else
+inline constexpr bool kVrPassthroughDefault = true;
+#endif
+
+inline bool VrPassthrough(bool fallback = kVrPassthroughDefault) {
     return Get().vrPassthrough.value_or(fallback);
 }