DKR-R v1.0.9 - Custom music preview repair

This fixes the safe stop while highlighting a custom track in Online:
"Custom music was not prepared at a confirmed loading boundary."

The inline Track Select preview was missing the silent music handling used
by deferred background previews. It tried to acquire a full recording while
still inside a menu tick. Confirmed inline previews now suppress that music
through the same existing empty-library path. This suppression does not grant
decoder permission. The actual race still prepares and verifies its selected
music during confirmed scene loading. Audio mixing and checkpoint restore
cannot open a decoder. No track reimport or save change is required.

All participants need this complete 1.0.9-music-playtest.1 build. Its private
course-prepare service changes the agreed peer identity; do not mix it with
the earlier 1.0.9 release. Public lobbies, spectators, swaps, connection checks,
custom cars and the existing launcher design remain intact. The website/API
requires no redeployment for this fix.

Please check the same online setup and custom track that failed:
- Move between custom tracks with recorded music in Track Select.
- Confirm the selected track and race; its custom music should play.
- Finish, return to Track Select, select another track and start again.
- Try a retry, a stock track and spectator viewing if available.
- Use both supported US ROM revisions and Windows/Linux peers if available.

Compilation/linking and source/package checks are separate from gameplay.
No game, mod, native test executable or AppImage is run by these tools.
Keep the complete package together. No ROMs, personal saves or mods included.
This is a repair playtest, not a published or auto-update-signed release.
