Install script: build and install Retro Rewind with --retro-rewind

The Steam Frame install script built only the base game. With
--retro-rewind it now fetches the Retro Rewind pack from Retro Rewind's
update server, as the Quest app and Wheel Wizard do: the full download
once, then each published update and its deletions. It also fetches the
Retro-WFC payload online play needs, builds both products from one
translation, copies the pack beside the disc on the Frame, sets
[paths] retro_rewind_root and adds RetroRewind to the Steam library.
--retro-rewind-pack uses a RetroRewind6 folder the player already has.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Sabhzu6VgoWcHtpSk4C9KZ

README.md

@@ -90,6 +90,12 @@ curl -fsSL https://raw.githubusercontent.com/mitch030504/Wiicompiled_VR_Frame/op
   It is added to your Steam library through Valve's devkit tools in `~/devkit-utils`, which Frame
   Control and Valve's Devkit Client put there. Without them, the script says how to add it once
   yourself. Steam must be running on the Frame for this step.
+- Add `--retro-rewind` to also build [Retro Rewind](https://wiki.tockdom.com/wiki/Retro_Rewind).
+  The script downloads its pack (about 4 GB) from Retro Rewind's own update server, as Wheel Wizard
+  does, and later runs apply only the updates published since. It also fetches the Retro-WFC
+  payload online play needs. On the Frame, Retro Rewind goes to `~/devkit-game/RetroRewind/` and
+  its pack to `~/wiicompiled/RetroRewind6`, and it shares WiiCompiled's settings.
+  `--retro-rewind-pack DIR` uses a RetroRewind6 folder you already have instead.
 - `--help` lists every option. From a downloaded release, run `Launcher/steam-frame-install.sh`
   with the same options: it then builds that release's source.