x86-64 client: SteamRT3 opt-in marker, and the arm64 parts on its build

Found porting to Bannerlator:
- The client opts into its SteamRT3 build only with a file,
  .steam-enable-steamrt64-client, in its folder; without it it
  re-executes in the scout environment and the session stalls before
  the interface. This install had one made by hand during bring-up;
  bannerlator-steam-x64 now writes it.
- The arm64 web helper only talks to a client of its own build, and the
  x86-64 client updates itself on every start while the arm64 one only
  when it runs (1790721607 against 1790904859: helper restarted every
  10 s). bannerlator-steam-install --sync CHANNEL brings the arm64 client
  to the channel's current build (the session runs it in x86-64 mode),
  and the arm64 helper is used only while both manifests name one
  build; otherwise the client keeps its own x86 helper.

tools/linuxfs/overlay/usr/local/bin/bannerlator-session

@@ -264,6 +264,9 @@ case $mode in
       # its game overlay and steamclient.so serve the arm64 Protons the x86-64 client starts.
       step "checking the Steam client's arm64 parts (first run downloads them, ~1-2 min)"
       bannerlator-steam-install || exit $?
+      # Its web helper only talks to a client of its own build; the x86-64 client updates itself on
+      # every start, the arm64 one only when it runs.
+      bannerlator-steam-install --sync "${BL_STEAM_CHANNEL:-publicbeta}" || step "could not update the arm64 client's parts; the x86-64 client keeps its own web helper"
       step "checking the x86-64 Steam client (FEX, experimental; the first run downloads ~2 GB)"
       bannerlator-steam-x64 --prepare || exit $?
       step "x86-64 Steam client ready"

tools/linuxfs/overlay/usr/local/bin/bannerlator-steam-install

@@ -1,17 +1,35 @@
 #!/bin/bash
 # Fetches the native arm64 Steam client from Valve's client-update CDN into ~/.local/share/Steam.
 # The client is not redistributable, so the runtime ships this script instead of the client.
+#
+#   bannerlator-steam-install                   install once
+#   bannerlator-steam-install --sync CHANNEL    bring the installed client to CHANNEL's current build
+#
+# --sync is for the x86-64 client option (bannerlator-steam-x64): it runs this client's web helper,
+# which only talks to a client of its own build, and the x86-64 client updates itself on every start
+# while this one only updates when it runs (device: 1790721607 against 1790904859, the helper restarted
+# every 10 s and the interface never came up).
 set -eu
 cdn=https://client-update.fastly.steamstatic.com
 manifest=steam_client_publicbeta_linuxarm64
 steam_root=${XDG_DATA_HOME:-$HOME/.local/share}/Steam
 stamp=$steam_root/package/bannerlator-installed
-[ -x "$steam_root/steamrtarm64/steam" ] && [ -f "$stamp" ] && exit 0
+sync=
+if [ "${1:-}" = --sync ]; then
+  case "${2:-}" in publicbeta|steamdeck_publicbeta) sync=$2 ;; *) sync=publicbeta ;; esac
+  manifest=steam_client_${sync}_linuxarm64
+fi
+[ -z "$sync" ] && [ -x "$steam_root/steamrtarm64/steam" ] && [ -f "$stamp" ] && exit 0
 
 work=$(mktemp -d)
 trap 'rm -rf "$work"' EXIT
 curl -fsSL "$cdn/$manifest" -o "$work/manifest"
 version=$(awk -F'"' '$2 == "version" { print $4; exit }' "$work/manifest")
+if [ -n "$sync" ]; then
+  installed=$(awk -F'"' '$2 == "version" { print $4; exit }' "$steam_root/package/$manifest.manifest" 2>/dev/null || true)
+  [ -x "$steam_root/steamrtarm64/steam" ] && [ "$installed" = "$version" ] && exit 0
+  echo "== STEP $(date -u +%H:%M:%S) updating the arm64 Steam client to $version (was ${installed:-none}) for the x86-64 client"
+fi
 # Every component block: its name, then the plain zip's file name and sha256.
 awk -F'"' '
   /^\t"/ && NF >= 3 && $4 == "" { block = $2 }
@@ -41,7 +59,12 @@ find "$steam_root" -depth -name '*\\*' | while read -r odd; do
     mv -f "$odd" "$fixed"
   fi
 done
-echo publicbeta > "$steam_root/package/beta"
+if [ -n "$sync" ]; then
+  # The client's own record of what it has, so it does not take the new files for an update to undo.
+  cp -f "$work/manifest" "$steam_root/package/$manifest.manifest"
+else
+  echo publicbeta > "$steam_root/package/beta"
+fi
 echo "$version" > "$stamp"
 ln -sfn "$steam_root/steamrtarm64" "$steam_root/steamrtarm32"
 chmod +x "$steam_root/steamrtarm64/steam" 2>/dev/null || true

tools/linuxfs/overlay/usr/local/bin/bannerlator-steam-x64

@@ -61,6 +61,10 @@ if [ ! -x /usr/local/bin/FEX ] || [ "$(cat /usr/local/share/fex-emu/.droiddeck-t
   mkdir -p /usr/local/share/fex-emu && echo "$FEX_TAG" > /usr/local/share/fex-emu/.droiddeck-tag
 fi
 step "FEX ready ($FEX_TAG)"
+# The client takes this file in its folder as the opt-in to its SteamRT3 build (steamrt64/steam).
+# Without it, it re-executes itself in the scout environment through steam.sh, whose start-up checks
+# need what proot cannot give, and the session stalls before the interface (device, Bannerlator).
+mkdir -p "$STEAM_X64" && : > "$STEAM_X64/.steam-enable-steamrt64-client"
 
 # --- x86 rootfs --------------------------------------------------------------------------------
 if [ ! -f "$ROOTFS/.droiddeck-complete" ]; then
@@ -561,9 +565,20 @@ x64_exec() {
   # program to the CPU): the session shim turns steamrt64/steamwebhelper.sh's exec of ./steamwebhelper
   # into it (preload/net.c). Nothing of the client's is edited - it checksums its files as it starts
   # and reinstalled itself over an edited launcher (2-3 minutes) on every start.
+  # Only while both clients are one build: the helper talks to the client over IPC, and one of another
+  # build never comes up - the client restarted it every 10 s (device: 1790721607 against 1790904859).
+  # Otherwise the client keeps its own (x86, software) helper. The session updates the arm64 parts first.
   if [ "${BL_X64_X86_WEBHELPER:-0}" != 1 ] && [ -x /usr/local/bin/bannerlator-x64-arm-webhelper ] &&
      [ -x "$ARM_STEAM/steamrtarm64/steamwebhelper" ]; then
-    export BL_X64_ARM_WEBHELPER=/usr/local/bin/bannerlator-x64-arm-webhelper
+    local chan x86_build arm_build
+    chan=$(cat "$STEAM_X64/package/beta" 2>/dev/null || echo publicbeta)
+    x86_build=$(awk -F'"' '$2 == "version" { print $4; exit }' "$STEAM_X64/package/steam_client_${chan}_ubuntu12.manifest" 2>/dev/null)
+    arm_build=$(awk -F'"' '$2 == "version" { print $4; exit }' "$ARM_STEAM/package/steam_client_${chan}_linuxarm64.manifest" 2>/dev/null)
+    if [ -n "$x86_build" ] && [ "$x86_build" = "$arm_build" ]; then
+      export BL_X64_ARM_WEBHELPER=/usr/local/bin/bannerlator-x64-arm-webhelper
+    else
+      echo "== x86-64 client build ${x86_build:-?}, arm64 web helper's ${arm_build:-?}: keeping the x86 web helper" >&2
+    fi
   fi
   exec "${BL_FEX:-/usr/local/bin/FEX}" "$STEAM_X64/steamrt64/steam" "$@"
 }