Move all buzzer/melody dispatch (DM, channel, advert) into a new
SoundNotifier class in SoundNotifier.h, isolating fork-specific sound
code so upstream changes to notify() no longer cause merge conflicts.
The shared playSlot() helper also removes the custom_played
duplication that existed across the three cases.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
