Summary

• Request Betaflight MSP_MOTOR_TELEMETRY (139) after FC init and keep mechanical RPM for motors 0..3 (quad only).
• Extend the existing 0xFF service payload from 15 to 21 bytes with four 12-bit quantized RPMs (6 bytes). No new RF opcodes; MSP RX buffer stays 64 bytes.
• Document the wire format in docs/synthetic-motor-audio.md. Pair with the matching goggle PR for decode/synth.

Packet

• Quantize: q = min((rpm + 16) >> 5, 4095) (32 RPM steps, max 131040).
• Legacy fields unchanged; old goggles that honor length accept the longer frame and ignore the unknown tail.

Test plan

• Flash VTX with BF FC that exposes motor telemetry; confirm MSP 139 is requested only for BTFL.
• Verify 0xFF service length is 21 and CRC still validates on current goggle firmware.
• Pair with goggle PoC branch and confirm RPM updates ~8 Hz / audio reacts.