Add double-press mute button for HCR audio

[thePunderWoman]

Summary

• Adds mutebutton=N config option (1–9, same button numbering as altbtn) that toggles HCR mute on double-press
• Mute sets all three HCR channels to 0 while remembering the live per-channel volumes (reflecting any wheel adjustments since boot)
• Unmute (second double-press) restores those saved volumes
• Moving the volume wheel while muted automatically unmutes — restores saved volumes, then applies the wheel value to the configured channel(s)
• May be set to the same button as altbtn — a quick double-tap fires mute while holding acts as the alt modifier
• Double-press window is 400ms (AmidalaController::DOUBLE_PRESS_MS)
• No-op when mutebutton=0 (default, disabled)

Changed files

• include/params.h — mutebutton field + default
• src/config.cpp — parse mutebutton=
• include/audio.h / src/audio.cpp — fSavedVolV/A/B tracks live channel volumes; restoreVolumes() helper; toggleMute(); setVolumeNoResponse / setAltVolumeNoResponse unmute on wheel input
• include/controller.h — noteMuteBtnUp() double-press timer logic; resetMutePressTimer(); toggleMute() forwarder; DOUBLE_PRESS_MS constant; fLastMuteBtnUpTime state
• src/drive_controllers.cpp — double-press detection wired into both DriveController (buttons 1–5) and DomeController (buttons 6–9); timer reset on disconnect
• example_config.txt — documents mutebutton= near altbtn=

Test plan

• pio test -e native — all 448 tests pass (new tests in test_audio and test_processconfig)
• pio run — firmware builds clean
• On hardware: confirm double-tapping the configured button mutes all audio
• On hardware: confirm second double-tap restores volumes correctly
• On hardware: confirm wheel movement while muted unmutes and adjusts volume
• On hardware: confirm mutebutton == altbtn — hold fires alt modifier, quick double-tap fires mute

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