Add power periph event diagnostics trigger API

[hello-shivam-bhardwaj]

Summary

This is the small stretch4_body companion PR for the firmware diagnostics gate in hello-robot/stretch_firmware_ii#32.

It does not change normal robot behavior by itself. It only exposes the host-side controls needed to explicitly enable or disable the firmware diagnostic window when debugging.

What Changed

• Added power-periph trigger constants for firmware event diagnostics enable/disable.
• Added direct PowerPeriph methods to enable and disable diagnostics.
• Added RobotClient queue methods for the same operations through the body server path.
• Documented the workflow in the status log primer.
• Added a focused stdlib unit test for the trigger bits and queued client methods.

Risk Controls

• Diagnostics remain off by default in firmware.
• The body API only sends explicit trigger bits; it does not enable background polling or logging on its own.
• The direct and body-server paths use the same trigger names so debug tooling can stay consistent.

Validation

Local:

• python3 test/test_power_periph_event_diagnostics.py

Bench / NUC:

• Installed public hello-robot-stretch4-body on the NUC.
• Verified stretch_body_ii is no longer imported by active console scripts.
• Verified direct battery status through /dev/hello-power-periph.
• Verified the diagnostic trigger methods are present in the installed package.

Suggested Review Focus

This PR is mainly API plumbing. The useful review questions are whether the trigger names, direct API, body-server API, and docs are clear enough for debug tooling and reviewers.