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.cpp

@@ -0,0 +1,19 @@
+#include "rpm.h"
+#include <Arduino.h>
+
+// Mock RPM: ramps up/down between idle and redline
+static int _rpm = 800;
+
+void rpm_init() {
+  _rpm = 800;
+}
+
+void rpm_update() {
+  // ~100ms per call at 10Hz = takes ~7s to sweep range
+  _rpm += 10;
+  if (_rpm >= 8000) { _rpm = 800;}
+}
+
+int rpm_get() {
+  return _rpm;
+}