Add WebXR Chromium build and install scripts

Flathub Chromium can't enter immersive WebXR on Linux because upstream
only wires the OpenXR device on Windows. Document why, and add scripts to
cross-compile arm64 Chromium with the unmerged Linux OpenXR CLs and to
install, launch and check it on the Frame.

The first build is still running; immersive-vr support is unverified.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/webxr-chromium.md

@@ -0,0 +1,82 @@
+# WebXR in Chromium on the Frame
+
+Goal: open a web VR180 or 360 player (DeoVR and DL8 embeds, WebXR samples),
+press its VR button, and watch in 3D in the headset.
+
+## Why Flathub Chromium can't
+
+**Verified 2026-09-25** (Frame BUILD_ID 20260922.6101926, Flathub
+`org.chromium.Chromium` 154.0.8037.57 aarch64):
+
+- `navigator.xr` exists, but `isSessionSupported("immersive-vr")` is `false`.
+  Flags don't change that, and neither does `--force-webxr-runtime=openxr`
+  with SteamVR exposed to the Flatpak.
+- The binary has no OpenXR loader: no `XR_RUNTIME_JSON`,
+  `xrGetInstanceProcAddr` or `XR_LOADER_DEBUG` strings. The only OpenXR
+  strings are the `chrome://flags` entries.
+
+**Cause (verified against Chromium source, same date).** M154 is the first
+release that compiles OpenXR on Linux:
+
+- `device/vr/buildflags/buildflags.gni` adds `is_linux` to `enable_openxr`.
+- Flathub's tarball sets `checkout_openxr = true`.
+- Flathub's GN args don't turn it off.
+
+But `content/services/isolated_xr_device/xr_runtime_provider.cc` only creates
+the OpenXR device under `ENABLE_OPENXR && IS_WIN`. That's true on 154, 155 and
+`main`. Nothing on Linux calls the OpenXR code, so the linker drops it. The
+rest of the Linux port is in two unmerged CLs (bug 506004811):
+
+- [8441736](https://chromium-review.googlesource.com/c/chromium/src/+/8441736)
+  runs the XR device service in a Linux sandbox that allows SteamVR's
+  sockets, `/dev/shm` and `flock`.
+- [8132979](https://chromium-review.googlesource.com/c/chromium/src/+/8132979)
+  wires the provider to `OpenXrPlatformHelperLinux`. `kOpenXR` stays off by
+  default, so it needs `--enable-features=OpenXR`.
+
+8132979 sits on top of 8441736, so fetching `refs/changes/79/8132979/<ps>`
+gets both.
+
+**The Frame side is ready.** `~/.config/openxr/1/active_runtime.json` names
+SteamVR (`bin/linuxarm64/vrclient.so`, `VALVE_runtime_is_steamvr`). The
+Linux backend uses Vulkan (`XR_USE_GRAPHICS_API_VULKAN`).
+
+## Building it
+
+[`scripts/build-chromium-xr.sh`](../scripts/build-chromium-xr.sh)
+cross-compiles arm64 Linux Chromium on an x64 Linux host. It doesn't need
+sudo: the arm64 sysroot comes from Chromium's own script. It needs about
+90 GB of disk. It shallow-fetches the CL ref, runs `gclient sync --no-history`,
+installs the sysroot, builds `chrome` with `symbol_level=0` and proprietary
+codecs, and packs `chromium-xr-arm64.tar.xz`. Progress is logged to
+`~/chromium-xr/stage`. The build aborts if `/` drops below 12 GB free.
+
+First run: buildhost (12 cores, 31 GB RAM), started 2026-09-25.
+
+## Running it on the Frame
+
+[`scripts/chromium-xr.sh`](../scripts/chromium-xr.sh):
+
+```sh
+scripts/chromium-xr.sh install          # scp from buildhost, unpack to ~/chromium-xr
+scripts/chromium-xr.sh launch [URL]     # headset desktop, --enable-features=OpenXR
+scripts/chromium-xr.sh check            # prints isSessionSupported('immersive-vr')
+```
+
+It runs natively, not as a Flatpak, so the XR sandbox and SteamVR's IPC work
+as the CL expects. It uses its own profile (`~/.config/chromium-xr`) and
+DevTools on loopback port 9223, so it doesn't collide with the Flatpak's 9222.
+
+**Verified 2026-09-25:**
+
+- Vulkan is there: Turnip (Mesa) on Adreno 750, API 1.4.359.
+- Unprivileged user namespaces work (`unshare -Ur true`), so Chromium's
+  namespace sandbox shouldn't need the setuid `chrome_sandbox`.
+
+**Unverified (inferred):**
+
+- Chromium's GPU process may still fall back from Vulkan to GL on Turnip.
+- An immersive session started from a window on the nested desktop may not
+  hand over cleanly to the SteamVR compositor.
+- If the sandbox fails to start, `--no-sandbox` is the fallback for a first
+  test.