2.2.9: fix shutdown drain from P0.6

[TobiasRoeddiger]

PPG initialization drives P0.6/IO4 high and leaves it high after shutdown. On the Main v2.7/flex assembly, PPG LDO_EN is supplied by V_LS; driving IO4 can back-power the switched rail and cause excessive off-state drain.

Remove that GPIO output configuration so P0.6 remains in its reset high-impedance state. Keep the existing PPG load-switch controls and startup delay.

This is the focused 2.2.9 release. Its diff against main contains only:

• The P0.6 fix in src/SensorManager/PPG.cpp, extracted unchanged from fix(ppg): leave P0.6 disconnected #290.
• VERSION: 2.2.8 → 2.2.9.

The PPG rate improvements, Bosch/BMA580 FIFO changes, audio/sensor fixes, throughput work and gauge interval change belong to the separate 2.2.10 development line.

Validation

• Verified the release diff contains exactly those two files and passes git diff --check.
• The fix is the same GPIO change previously tested in fix(ppg): leave P0.6 disconnected #290: instrumented 2.2.8 measurements fell from approximately 1,791 µA to 38 µA; after the single-ear Android tester sequence, the final 120-second off-state capture averaged 38.16 µA.
• Those are archived measurements of the original diagnostic validation, not a new hardware run of this release branch. Manual acoustic/fixture confirmations were assumed and do not constitute a manufacturing pass.
• A reset during the firmware update clears the old GPIO output configuration. This patch does not actively rewrite the pin on an already-running old image.

The production firmware in the current main matches v2.2.8 before this patch; later main-only documentation/tool changes are inherited without adding other firmware fixes. No additional regression tests are added.

The broader release is tracked in #295 on branch 2.2.10.

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✅ Unit tests passed

2 passed, 0 failed/error, 0 skipped — view workflow run

Test scenario	Platform	Result
sensor_component/openearable.unit.sensor_component	native_sim/native/64	✅ passed
ring_buffer/openearable.unit.ring_buffer	native_sim/native/64	✅ passed

Download the unit-test-results artifact for full Twister reports and logs.

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Compiler warnings

The extended-warning build completed successfully.

Application compiler warnings

None.

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Build output available:
openearable_v2_firmware.elf.zip
openearable_v2_fota.zip

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CodeChecker static analysis

✅ No non-style issues found.