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/rpm.h

@@ -0,0 +1,8 @@
+#ifndef RPM_H
+#define RPM_H
+
+void rpm_init();
+void rpm_update();      // Call in loop
+int rpm_get();          // Returns current RPM (0 if invalid)
+
+#endif