Move the PC-streaming layer into pc-streaming/, apart from the Steam Frame build

The PC compositor-layer work (the Direct3D 11 proxy, its installer, its notes) now lives in pc-streaming/, clearly
separate and marked experimental, so the main project is the Steam Frame build and the fixes that help it. The PC
layer can't change the Steam Frame's home (the Steam Frame renders it on the headset), and this keeps it from
crowding the Steam Frame release while preserving it for a tethered PC headset.

The shared Windows renderer support (src/, behind #ifdef _WIN32 / FRAME_HOME_PC) stays put: it only compiles into a
PC build and never touches the Steam Frame build. Build and README references updated to the new paths.

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

scripts/build-studio.sh

@@ -72,8 +72,8 @@ stage_pc() {
     case "$target" in
         windows-x64) mkdir -p "$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/" ;;
+                     cp "$ROOT/pc-streaming/install-frame-home-pc.ps1" "$ROOT/pc-streaming/Install Frame Home on this PC.bat" \
+                        "$ROOT/pc-streaming/Remove Frame Home from this PC.bat" "$stage/pc/" ;;
         linux-x64)   mkdir -p "$stage/pc/bin" "$stage/pc/layer"
                      cp "$home/frame-home" "$home/frame-switcher" "$stage/pc/bin/"
                      ${CXX:-g++} -O2 -shared -fPIC -std=c++17 -fvisibility=hidden -I"$ROOT/third_party/vulkan-headers/include" -I"$ROOT/src" \