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>

scripts/chromium-xr.sh

@@ -0,0 +1,97 @@
+#!/usr/bin/env zsh
+# Mac-side: install and launch the WebXR-enabled Chromium build on the Frame.
+#
+# Flathub Chromium can't enter immersive WebXR on Linux: upstream only wires
+# the OpenXR device on Windows (see docs/webxr-chromium.md). This deploys an
+# arm64 build with the Linux OpenXR CLs, made on a Linux host by
+# scripts/build-chromium-xr.sh, into ~/chromium-xr on the Frame (not a
+# Flatpak, so SteamVR's sockets and the XR sandbox work unmodified).
+#
+# Usage:
+#   scripts/chromium-xr.sh install [TARBALL]  # default: fetch from $BUILD_HOST
+#   scripts/chromium-xr.sh launch [URL]       # opens in the headset desktop
+#   scripts/chromium-xr.sh check              # isSessionSupported via DevTools
+set -euo pipefail
+
+FRAME_ALIAS=${FRAME_ALIAS:-frame}
+BUILD_HOST=${BUILD_HOST:-buildhost}
+BUILD_TARBALL=${BUILD_TARBALL:-chromium-xr/chromium-xr-arm64.tar.xz}
+DEVTOOLS_PORT=${DEVTOOLS_PORT:-9223}
+here=${0:A:h}
+
+case "${1:-}" in
+  install)
+    tarball=${2:-}
+    if [[ -z "$tarball" ]]; then
+      tmp=$(mktemp -d)
+      trap 'rm -rf "$tmp"' EXIT
+      tarball=$tmp/chromium-xr-arm64.tar.xz
+      scp -q "$BUILD_HOST:$BUILD_TARBALL" "$tarball"
+    fi
+    ssh "$FRAME_ALIAS" 'rm -rf ~/chromium-xr.new && mkdir -p ~/chromium-xr.new'
+    ssh "$FRAME_ALIAS" 'tar -xJf - -C ~/chromium-xr.new' < "$tarball"
+    # Check the new build runs before replacing the old one.
+    ssh "$FRAME_ALIAS" '~/chromium-xr.new/chrome --version && rm -rf ~/chromium-xr && mv ~/chromium-xr.new ~/chromium-xr'
+    ;;
+  launch)
+    # run-on-frame starts in $HOME on the Frame, so the profile path is relative.
+    exec "$here/run-on-frame.sh" -- '~/chromium-xr/chrome' \
+      --user-data-dir=.config/chromium-xr \
+      --enable-features=OpenXR \
+      --remote-debugging-port="$DEVTOOLS_PORT" \
+      "${2:-https://immersive-web.github.io/webxr-samples/}"
+    ;;
+  check)
+    # DevTools listens on the Frame's loopback only; evaluate there.
+    ssh "$FRAME_ALIAS" python3 - "$DEVTOOLS_PORT" <<'EOF'
+import json, sys, urllib.request, base64, os, socket, struct
+port = int(sys.argv[1])
+tabs = json.load(urllib.request.urlopen(f"http://127.0.0.1:{port}/json", timeout=10))
+page = next((t for t in tabs if t["type"] == "page"), None)
+if page is None:
+    sys.exit("no open page: run 'chromium-xr.sh launch' first")
+path = page["webSocketDebuggerUrl"].split(f":{port}", 1)[1]
+s = socket.create_connection(("127.0.0.1", port), timeout=30)
+key = base64.b64encode(os.urandom(16)).decode()
+s.sendall(f"GET {path} HTTP/1.1\r\nHost: 127.0.0.1\r\nUpgrade: websocket\r\n"
+       f"Connection: Upgrade\r\nSec-WebSocket-Key: {key}\r\n"
+       "Sec-WebSocket-Version: 13\r\n\r\n".encode())
+s.recv(4096)
+msg = json.dumps({"id": 1, "method": "Runtime.evaluate", "params": {
+    "expression": "navigator.xr ? navigator.xr.isSessionSupported('immersive-vr') : 'no navigator.xr'",
+    "awaitPromise": True}}).encode()
+mask = os.urandom(4)
+hdr = bytes([0x81]) + (bytes([0x80 | len(msg)]) if len(msg) < 126
+                       else bytes([0x80 | 126]) + struct.pack(">H", len(msg)))
+s.sendall(hdr + mask + bytes(b ^ mask[i % 4] for i, b in enumerate(msg)))
+buf = b""
+reply = None
+while reply is None:
+    chunk = s.recv(65536)
+    if not chunk:
+        sys.exit("DevTools closed the connection")
+    buf += chunk
+    # Consume every complete frame already buffered before reading again.
+    while len(buf) >= 2:
+        n = buf[1] & 0x7F
+        off = 2
+        if n == 126:
+            if len(buf) < 4:
+                break
+            n, off = struct.unpack(">H", buf[2:4])[0], 4
+        elif n == 127:
+            if len(buf) < 10:
+                break
+            n, off = struct.unpack(">Q", buf[2:10])[0], 10
+        if len(buf) < off + n:
+            break
+        frame, buf = buf[off:off + n], buf[off + n:]
+        msg = json.loads(frame)
+        if msg.get("id") == 1:
+            reply = msg
+            break
+print("immersive-vr supported:", reply["result"]["result"].get("value"))
+EOF
+    ;;
+  *) sed -n '2,13p' "$0"; exit 2 ;;
+esac