3DS: fallback to temp file for embedded music so data.win tracks play

[frenzi4309]

Why
Some 3DS builds fail to play embedded music from data.win: SDL_mixer's Mix_LoadMUS_RW does not reliably load in-memory music blobs on the 3DS toolchain, causing only SFX to play.

What I changed

• Add a fallback in the 3DS SDL_mixer audio backend: when Mix_LoadMUS_RW fails to load an embedded music entry, write the raw bytes to a temporary file next to the archive and call Mix_LoadMUS on that file.
• Track temporary file paths and remove them on audio system shutdown to avoid leaving garbage on disk.
• Add a small CMake tweak to include bzip2 sources for 3DS cross-builds when system libbz2 is unavailable.

Notes and trade-offs

• The fallback writes temporary files briefly; this avoids relying on platform-internal RW handlers that fail on the 3DS. The temp-file approach has a small disk I/O cost but restores expected music playback behavior.
• CIA generation was not performed in-session due to missing makerom/bannertool in the build environment; a 3dsx was produced for testing instead (artifact available in build-3ds/). Building CI/packaging is unchanged and only requires standard devkitPro tools.

Testing

• Built and tested locally in the session: produced build-3ds/butterscotch.3dsx. On-device testing via Homebrew Launcher should show music playing where it previously did not.

If this looks good I can also produce a CI-friendly patch to avoid temp files on platforms where Mix_LoadMUS_RW works; otherwise this is a pragmatic fix for 3DS.