Add per-channel HCR volume config and alt+wheel routing

[thePunderWoman]

Summary

• Adds volumeChA= and volumeChB= config options to set independent startup volumes for HCR channels A and B (useful when wav files are significantly louder than the voice channel)
• Adds volumewheel= to select which channel (global/voice/chA/chB) the volume wheel (L1 trigger) controls
• Adds altvolumewheel= to select a different target channel when the alt button is held while turning the wheel; 0 falls through to normal volumewheel behaviour
• All four options are HCR-only and no-ops on VMusic builds

Changed files

• include/params.h — 4 new fields + defaults
• src/config.cpp — parse the 4 new config keys
• include/audio.h / src/audio.cpp — per-channel startup init, applyHCRVolume() helper, setVolumeNoResponse routes by volumewheel, new setAltVolumeNoResponse
• include/controller.h — setAltVolumeNoResponse forwarder
• src/drive_controllers.cpp — L1 trigger checks isAltHeld() and dispatches to alt or normal volume path
• example_config.txt — documents all 4 new keys with examples

Test plan

• pio test -e native — all 18 suites pass (new tests in test_audio and test_processconfig)
• pio run — firmware builds clean
• On hardware: confirm volumeChA=30 starts channel A quieter than voice
• On hardware: confirm volumewheel=1 scopes the wheel to voice only
• On hardware: confirm altvolumewheel=2 + alt held adjusts chA while normal wheel adjusts according to volumewheel
• On hardware: confirm omitting the new keys gives identical behaviour to before

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