FirmMux SD setup

See docs in the repo for full details:
- docs/SD Layout.md
- docs/RetroArch Emulators.md
- docs/NDS Options.md

Quick paths:
- Autoboot template (used by Settings toggle): /3ds/FirmMux/boot.3dsx
- Autoboot metadata template (name/icon): /3ds/FirmMux/boot.smdh
- FirmMux: /3ds/FirmMux.3dsx
- Optional FirmMux HOME Menu icon: /cias/FirmMux.cia
- RetroArch (FirmMux build): /3ds/FirmMux/emulators/retroarch.3dsx
- RetroArch data/cores: /retroarch/
- External RetroArch configs: /3ds/emulators/
- RetroArch per‑ROM options: /3ds/emulators/rom_options.json (set via Y on a ROM)
- Optional BGM (loops): /3ds/FirmMux/bgm/bgm.wav
- NDS options: /_nds/firmmux/nds_options/
- NDS cheats: /_nds/ntr-forwarder/usrcheat.dat
- Note: NDS cheats are currently WIP; selection/flag writes are implemented, but in-game effect is not fully reliable yet.
- NDS widescreen bins: /_nds/firmmux/nds_widescreen/
- NDS launcher 3DSX (default): /3ds/FirmMux/firmux-bootstrap-prep.3dsx
- NDS launcher CIA: FirmMuxBootstrapLauncher (TitleID CTR-P-FMBP)
- Settings: “Check NDS launcher” (CIA) and “NDS launcher mode” (Auto/CIA/3DSX)
- NTR Forwarder releases (required baseline for NDS forwarder flow): https://github.com/RocketRobz/NTR_Forwarder/releases
- NTR Launcher releases (only the .cia is needed): https://github.com/ApacheThunder/NTR_Launcher/releases
- YANBF releases (install bootstrap.cia; needed for firmux-bootstrap-prep handoff): https://github.com/YANBForwarder/YANBF/releases

Widescreen system requirement:
- Install TWPatch and enable Widescreen, then patch via Select.
  https://gbatemp.net/download/twpatch.37400/history
- Enable Luma: External FIRM and modules.
- Move TwlBg.cxi from sd:/luma/sysmodules/ to sd:/_nds/ntr-forwarder/Widescreen.cxi