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/voltage.cpp

@@ -10,6 +10,7 @@ static const int PIN_VBAT = A0;
 static const float DIVIDER_RATIO = 47.0 / (100.0 + 47.0);  // ~0.3197
 static const float ADC_REF = 5.0;
 static const int ADC_MAX = 1023;
+static const float OFFSET = 0.2; // calib
 
 void voltage_init() {
   // analogRead doesn't need explicit pinMode, but here for future config
@@ -23,5 +24,5 @@ int voltage_read_raw() {
 float voltage_read() {
   int raw = voltage_read_raw();
   float vDivider = (raw / (float)ADC_MAX) * ADC_REF;
-  return vDivider / DIVIDER_RATIO;
+  return vDivider / DIVIDER_RATIO + OFFSET; 
 }