arduino: TSV telemetry protocol with mock RPM/gear

- null-terminated TSV frame: V_bat, IMU (9 fields), RPM, gear
- mock RPM ramps 800-8000, gear derived from RPM bands
- voltage calibration offset
- PROTOCOL.md documents wire format

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

arduino/main/gear.h

@@ -0,0 +1,7 @@
+#ifndef GEAR_H
+#define GEAR_H
+
+void gear_init();
+int gear_get(int rpm);  // Returns gear 0-6 (0=neutral)
+
+#endif