Frame Home as the real SteamVR environment on a PC (Windows too), not a home app

On the Steam Frame a layer inside SteamVR's compositor draws the home where it would draw Aurora, so the home is the
environment itself: at boot, during game loads, behind the Steam menu, with the controllers and passthrough, and it
steps aside for a game the instant it opens. On a PC this was faked with SteamVR's home-app slot and a flat 360 photo
in the gaps, which launched and handed off like an app and showed a picture, not the live home. This does it properly
on any PC that runs SteamVR, the Frame's way:

- Windows: the compositor is Direct3D 11, so the layer is a proxy d3d11.dll beside the compositor (layer/
  frame_home_layer_d3d11.cpp, built from the .def that forwards every other export to the real d3d11.dll, kept as
  fh_d3d11real.dll). It wraps the compositor's device, finds Aurora's draws by their shader bytecode and draws the
  home over the sky, leaving the floor and stars off; everything else passes straight through. Pure pass-through in
  any process that isn't vrcompositor.
- The home's frames cross from Frame Home (Vulkan) to the compositor (D3D11): the layer owns kSlots shared textures
  and hands Frame Home their NT handles over a named pipe; Frame Home imports them (VK_KHR_external_memory_win32) and
  renders into them. No cross-API lock — a slot is published only once the GPU has finished it and the four rotate,
  as on Linux. The shared State stays Frame Home's.
- The Environments panel runs frame-home --system and keeps it alive (one at a time, a pid file across panel
  restarts), the same systemEnv path as the Frame. The installer places/removes the proxy and marks the layer with a
  root/layer folder, in place of the old home-app registration and background pictures.
- Safe like the Vulkan layer: no home, or one stale over half a second, and the compositor draws Aurora as usual;
  the layer turns itself off after two unhealthy compositor starts, and root/layer/off turns it off by hand.

Linux keeps its Vulkan layer (unchanged). Verified without a headset on the null driver: the proxy loads and wraps
the compositor without crashing, recognises Aurora's shaders, and the frame hand-over completes (Frame Home connects,
imports the textures, renders). The compositor only draws the environment while presenting to a headset, so the final
on-headset image is the one step the null driver can't exercise.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

scripts/build-studio.sh

@@ -60,17 +60,18 @@ build_pc_home() {
     say "building Frame Home for $target's SteamVR"
     "$CMAKE" -S "$ROOT" -B "$dir" -DCMAKE_BUILD_TYPE=Release -DFRAME_HOME_PC=ON "$@" > "$dir.configure.log" 2>&1 || {
         tail -20 "$dir.configure.log"; say "$target: Frame Home configure failed (full log: $dir.configure.log)"; exit 1; }
-    "$CMAKE" --build "$dir" --target frame-home frame-switcher --parallel
+    "$CMAKE" --build "$dir" --target frame-home frame-switcher $([ "$target" = windows-x64 ] && echo d3d11.dll) --parallel
 }
 
-# pc/ beside Studio: Frame Home for this computer's SteamVR and its installer (Studio's Headset tab runs it).
-# Windows: SteamVR's home app (install-frame-home-pc.ps1). Linux: the Steam Frame's way, the compositor layer, with
+# pc/ beside Studio: Frame Home for this computer's SteamVR and its installer (Studio's Headset tab runs it). Both
+# platforms the Steam Frame's way now — the compositor layer draws the home in Aurora's place. Windows: the layer is
+# a proxy d3d11.dll (install-frame-home-pc.ps1 puts it beside SteamVR's compositor). Linux: the Vulkan layer, with
 # the headset's own install.sh.
 stage_pc() {
     target=$1 stage=$2 home="$BUILD/$1-home"
     case "$target" in
         windows-x64) mkdir -p "$stage/pc"
-                     cp "$home/frame-home.exe" "$home/frame-switcher.exe" "$stage/pc/"
+                     cp "$home/frame-home.exe" "$home/frame-switcher.exe" "$home/d3d11.dll" "$stage/pc/"
                      cp "$ROOT/frame/pc/install-frame-home-pc.ps1" "$ROOT/frame/pc/Install Frame Home on this PC.bat" \
                         "$ROOT/frame/pc/Remove Frame Home from this PC.bat" "$stage/pc/" ;;
         linux-x64)   mkdir -p "$stage/pc/bin" "$stage/pc/layer"