[Mycat-04]

Extend the ArmSoM CM5 source-build guide with a graphical main-menu launcher and direct desktop launchers for three homebrew games: Falling (NES/Nestopia), GBAtris 1.0 (GBA/mGBA), and Snake 2.0 (GB/mGBA). Each launcher uses the installed RetroArch icon and opens without a terminal.

The guide includes fixed-version upstream game downloads, ROM checksums, Debian mGBA core extraction and runtime dependency installation, source/license retention notes, and a concise keyboard reference. RetroArch itself remains source-built; emulator cores are prebuilt Debian packages.

Desktop setup handles missing xdg-user-dir and documents recovery from a root-owned installation to the XFCE desktop user. It checks required Ozone assets, enables mouse menu input, and sets the core/content browser directories.

Validation on native Debian 12 XFCE ARM64, CM5, October 27, 2026:

• Main-menu desktop launch, official icon, normal mouse pointer, mouse selection and keyboard back navigation verified.
• Falling, GBAtris and Snake: gameplay, keyboard controls, HDMI audio, exit and relaunch through each desktop icon confirmed by the board owner. GBAtris pause/resume was also checked.
• ROM SHA-256 checks and desktop-file-validate pass.
• All 14 Bash blocks pass bash -n; git diff --check passes.
• The complete site builds with mkdocs build --strict (exit code 0).

No RetroArch source changes or game binaries are included in this documentation PR.