PoC: synthetic motor audio from 0xFF RPM telemetry

[350d]

Summary

• Decode the optional 6-byte motor RPM tail on the HDZero 0xFF service packet (length >= 21).
• Synthesize stereo propeller audio at 22.05 kHz (QUADX pan, quieter rears); stale/zero RPM fades out.
• Record Option → Motor Audio (On/Off, record.motor_audio): live mix beside the selected Mic / Line In / A/V In source (opens DAC path; Softwinner AO PCM feed still TODO on hardware).
• Host tool utilities/motor_audio_poc for unit tests, synthetic WAV, and blackbox CSV replay. Docs in docs/synthetic-motor-audio.md.
• Pair with the matching VTX PR that appends the packed RPM bytes.

Notes

• Hardware goggles still need a Softwinner AO PCM sink into the DAC path; decoder + mute gate + out_dac_on are ready.
• Emulator/host can play via SDL or write WAV. Full macOS UI emulator needs the cross-platform SDL work from Add a cross-platform SDL emulator for developing the goggle UI without hardware #617.
• No hex/octo, no Betaflight changes.

Test plan

• cmake -S utilities/motor_audio_poc -B tmp/motor_audio_poc_build && cmake --build tmp/motor_audio_poc_build
• ./tmp/motor_audio_poc_build/motor_audio_poc --motor-test → PASS
• --motor-synthetic --motor-wav tmp/out.wav and listen
• --motor-replay <csv> --motor-poles N --motor-realtime (Meteor75 verified ~60s)
• UI: Record Option → Motor Audio On/Off persists in setting.ini
• Against VTX PoC: confirm 21-byte service enables audio; 15-byte legacy stays silent

[350d]

Demo audio generated from random blackbox log from internet:
motor_meteor75_replay.wav.zip

[rquellet]

I tested this yesterday on a live drone with headphones connected to the goggle. I see now that the live audio output function hasn't been implemented yet though. Looking forward to seeing this work, this is a project I've thought of doing myself in the past, and I'm happy to see someone work on it!