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/comms.h

@@ -2,6 +2,7 @@
 #define COMMS_H
 
 #include <Arduino.h>
+#include "imu.h"
 
 // Initialize Pi serial communication (call in setup)
 void comms_init();
@@ -10,7 +11,12 @@ void comms_init();
 // Returns true if a complete command was received
 bool comms_update();
 
-// Send telemetry line to Pi
+// Send complete telemetry frame (TSV format, null-terminated)
+// Format: V_bat\tAx\tAy\tAz\tGx\tGy\tGz\tRoll\tPitch\tYaw\tRPM\tGear\0
+// If imu_valid is false, IMU fields are empty (but tabs preserved)
+void comms_send_telemetry(float voltage, const ImuData& imu, bool imu_valid, int rpm, int gear);
+
+// Send key:value line (for debug/ACK, newline-terminated)
 void comms_send(const char* key, float value, int decimals = 2);
 void comms_send(const char* key, int value);
 void comms_send(const char* key, const char* value);