AP_Compass: add AF9838 backend

[Voltafield]

Summary

Adds an AP_Compass backend for the Voltafield AF9838 I2C magnetometer.

The driver supports AF9838 detection, initialization, single-measurement acquisition, magnetic XYZ conversion to milligauss, and probing through ArduPilot I2C bus paths.

Classification & Testing (check all that apply and add your own)

• Checked by a human programmer
• Non-functional change
• No-binary change
• Infrastructure change (e.g. unit tests, helper scripts)
• Automated test(s) verify changes (e.g. unit test, autotest)
• Tested manually, description below (e.g. SITL)
• Tested on hardware
• Logs attached
• Logs available on request

Description

Changes:

• Adds the AP_Compass_AF9838 backend.
• Adds DEVTYPE_AF9838 device identification and decode_devid.py support.
• Adds AF9838 compile-time configuration.
• Adds AF9838 I2C probing at address 0x0C.
• Supports external I2C buses and HAL I2C buses classified as internal, as required by the CubePurple connector used during testing.

Hardware tested:

• Flight controller: CubePurple
• Sensor: AF9838 external I2C module
• Interface: I2C
• I2C address: 0x0C
• CubeOrange: not tested

Build validation:

• ./waf clean: PASS
• ./waf configure --board CubePurple: PASS
• ./waf copter: PASS
• Final source commit: bc8f469d1ae4594011569a7819f5dcdf3c5823b7
• AP_Compass driver commit: 84e28adcf274c0301305425e53241be10abc8e3f

Commit history note:

• The original tested commit was 7ef6c16ecd0b3c9e1f85714322a9dadd0cd57faf.
• The commit was split into AP_Compass: and Tools: commits to satisfy ArduPilot branch-convention CI.
• The source tree before and after the split is identical; no source-code changes were introduced.

Hardware detection:

• COMPASS_DEV_ID: 1707009
• Decoded device type: DEVTYPE_AF9838
• COMPASS_EXTERNAL: 1
• Sensor detection and initialization: PASS

Raw magnetic XYZ validation:

• AF9838 was rotated through yaw, roll and pitch axes.
• X/Y/Z output changed continuously without stalls.
• Full RAW_IMU telemetry and extracted CSV are included in the test evidence.

Additional bench validation:

• Stationary stability test duration: 24 minutes 23 seconds
• Compass remained detected throughout the stability test.
• Three complete CubePurple power-cycle tests: PASS
• AF9838 was automatically detected after every restart.
• Final firmware was loaded and smoke-tested on CubePurple.

Test evidence:

• AF9838 build logs
• Firmware metadata and SHA-256
• Detection logs and device-ID decoding
• Raw magnetic XYZ CSV and telemetry logs
• Stability-test logs
• Power-cycle-test logs

AF9838_ArduPilot_Test_Evidence_CubePurple_20260729.zip