Frame Home on a PC with SteamVR (Windows, Linux); thumbnails from the spawn

- src/home_platform.h: what Frame Home and the panel need from the system (paths, finding and stopping each
  other, running a tool, restarting, priorities) for SteamOS, Windows and Linux PCs. On a PC the environments are
  Frame Studio's My Environments; Frame Home's own files in %LOCALAPPDATA%\Frame Home (Windows).
- src/openvr_loader.cpp: a PC build finds SteamVR's own OpenVR library at run time (openvrpaths.vrpath), so it runs
  on any PC with SteamVR; Vulkan through volk. CMake: FRAME_HOME_PC (default on Windows and x86-64 Linux); the Steam
  Frame build is unchanged.
- Windows: Frame Home is SteamVR's home app; SteamVR's classic environment shows a 360-degree picture of the current
  home from its spawn (frame-home --panorama --spawn, made by the panel when the home changes) instead of Aurora
  while Frame Home starts. Linux PCs: the Steam Frame's compositor layer, as on the headset.
- frame/pc: install-frame-home-pc.ps1 (+ .bat): online through SteamVR's API or offline in its config files; the
  SteamVR settings it changes are saved and restored exactly on -Uninstall (tested in a sandbox both ways).
  frame-switcher --uninstall. Studio: Properties > Headset > This PC installs it from pc/ beside Studio.
- Packaging and CI: pc/ in the Windows x64 and Linux x64 Studio downloads; FrameHome-PC-windows-x64.zip.
- Thumbnails come from where Frame Home starts you (the first spawn, eyes 1.6 m up): Studio's saves and the
  panel's (frame-home --snapshot --spawn).
- docs/pc-steamvr.md.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.github/workflows/build.yml

@@ -38,8 +38,36 @@ jobs:
           name: FrameStudio-linux-${{ matrix.arch }}
           path: dist/FrameStudio-linux-${{ matrix.arch }}.tar.gz
 
+  # Frame Home for SteamVR on a Windows PC (frame-home.exe, frame-switcher.exe and the PC installer), cross-built with
+  # llvm-mingw as it's developed: the pc/ folder of the Windows x64 Studio, and a download of its own.
+  pc-home-windows:
+    name: Frame Home for Windows PCs
+    runs-on: ubuntu-24.04
+    steps:
+      - uses: actions/checkout@v4
+      - name: llvm-mingw
+        run: |
+          sudo apt-get update && sudo apt-get install -y ninja-build cmake
+          url=$(curl -s https://api.github.com/repos/mstorsjo/llvm-mingw/releases/latest | grep -o 'https://[^"]*ucrt-ubuntu-[0-9.]*-x86_64\.tar\.xz' | head -1)
+          curl -sL "$url" | tar -xJ -C "$HOME" && mv "$HOME"/llvm-mingw-* "$HOME/llvm-mingw"
+      - name: Build
+        run: |
+          export PATH="$HOME/llvm-mingw/bin:$PATH"
+          cmake -S . -B build/pc-win64 -G Ninja -DCMAKE_BUILD_TYPE=Release -DFRAME_HOME_PC=ON -DCMAKE_TOOLCHAIN_FILE=cmake/llvm-mingw.cmake -DWINDOWS_ARCH=x86_64 -DLLVM_MINGW="$HOME/llvm-mingw"
+          cmake --build build/pc-win64 --target frame-home frame-switcher --parallel
+          mkdir -p "build/stage/FrameHome-PC-windows-x64" dist
+          cp build/pc-win64/frame-home.exe build/pc-win64/frame-switcher.exe frame/pc/install-frame-home-pc.ps1 \
+             "frame/pc/Install Frame Home on this PC.bat" "frame/pc/Remove Frame Home from this PC.bat" "build/stage/FrameHome-PC-windows-x64/"
+          cp LICENSE build/stage/FrameHome-PC-windows-x64/LICENSE.txt
+          (cd build/stage && zip -qr ../../dist/FrameHome-PC-windows-x64.zip FrameHome-PC-windows-x64)
+      - uses: actions/upload-artifact@v4
+        with:
+          name: FrameHome-PC-windows-x64
+          path: dist/FrameHome-PC-windows-x64.zip
+
   windows:
     name: Windows ${{ matrix.arch }}
+    needs: [pc-home-windows]
     strategy:
       fail-fast: false
       matrix:
@@ -47,6 +75,11 @@ jobs:
     runs-on: windows-latest
     steps:
       - uses: actions/checkout@v4
+      - uses: actions/download-artifact@v4
+        if: matrix.arch == 'x64'
+        with:
+          name: FrameHome-PC-windows-x64
+          path: pc
       - name: Build (MSVC)
         shell: cmd
         run: studio\build_windows.bat ${{ matrix.arch }} "%GITHUB_WORKSPACE%\build\windows-${{ matrix.arch }}"
@@ -59,6 +92,11 @@ jobs:
           Copy-Item "build/windows-${{ matrix.arch }}/Frame Studio.exe" $stage
           Copy-Item -Recurse "build/windows-${{ matrix.arch }}/frame" "$stage/frame"
           Copy-Item LICENSE "$stage/LICENSE.txt"
+          if ("${{ matrix.arch }}" -eq "x64") {   # Frame Home for this PC's SteamVR, beside Studio
+            Expand-Archive "pc/FrameHome-PC-windows-x64.zip" -DestinationPath build/pc
+            Copy-Item -Recurse "build/pc/FrameHome-PC-windows-x64" "$stage/pc"
+            Remove-Item "$stage/pc/LICENSE.txt"
+          }
           New-Item -ItemType Directory -Force dist | Out-Null
           Compress-Archive -Path $stage -DestinationPath "dist/$name.zip" -Force
       - uses: actions/upload-artifact@v4
@@ -102,7 +140,7 @@ jobs:
     name: Publish
     # (also when a platform failed: "latest" gets every build that worked; a version release needs them all)
     if: ${{ !cancelled() && (startsWith(github.ref, 'refs/tags/v') || github.ref == 'refs/heads/main') }}
-    needs: [linux, windows, macos, frame-home]
+    needs: [linux, windows, macos, frame-home, pc-home-windows]
     runs-on: ubuntu-24.04
     permissions:
       contents: write